diff --git a/x-pack/plugins/index_lifecycle_management/__jest__/client_integration/edit_policy/constants.ts b/x-pack/plugins/index_lifecycle_management/__jest__/client_integration/edit_policy/constants.ts
index 00c7d705c1f44a..68b2ac59d2a190 100644
--- a/x-pack/plugins/index_lifecycle_management/__jest__/client_integration/edit_policy/constants.ts
+++ b/x-pack/plugins/index_lifecycle_management/__jest__/client_integration/edit_policy/constants.ts
@@ -195,3 +195,41 @@ export const POLICY_WITH_NODE_ROLE_ALLOCATION: PolicyFromES = {
   },
   name: POLICY_NAME,
 };
+
+export const POLICY_WITH_KNOWN_AND_UNKNOWN_FIELDS = ({
+  version: 1,
+  modified_date: Date.now().toString(),
+  policy: {
+    foo: 'bar',
+    phases: {
+      hot: {
+        min_age: '0ms',
+        actions: {
+          rollover: {
+            unknown_setting: 123,
+            max_size: '50gb',
+          },
+        },
+      },
+      warm: {
+        actions: {
+          my_unfollow_action: {},
+          set_priority: {
+            priority: 22,
+            unknown_setting: true,
+          },
+        },
+      },
+      delete: {
+        wait_for_snapshot: {
+          policy: SNAPSHOT_POLICY_NAME,
+        },
+        delete: {
+          delete_searchable_snapshot: true,
+        },
+      },
+    },
+    name: POLICY_NAME,
+  },
+  name: POLICY_NAME,
+} as any) as PolicyFromES;
diff --git a/x-pack/plugins/index_lifecycle_management/__jest__/client_integration/edit_policy/edit_policy.test.ts b/x-pack/plugins/index_lifecycle_management/__jest__/client_integration/edit_policy/edit_policy.test.ts
index c91ee3e2a1c068..a203a434bb21a4 100644
--- a/x-pack/plugins/index_lifecycle_management/__jest__/client_integration/edit_policy/edit_policy.test.ts
+++ b/x-pack/plugins/index_lifecycle_management/__jest__/client_integration/edit_policy/edit_policy.test.ts
@@ -19,6 +19,7 @@ import {
   POLICY_WITH_INCLUDE_EXCLUDE,
   POLICY_WITH_NODE_ATTR_AND_OFF_ALLOCATION,
   POLICY_WITH_NODE_ROLE_ALLOCATION,
+  POLICY_WITH_KNOWN_AND_UNKNOWN_FIELDS,
   getDefaultHotPhasePolicy,
 } from './constants';
 
@@ -31,6 +32,70 @@ describe('<EditPolicy />', () => {
     server.restore();
   });
 
+  describe('serialization', () => {
+    /**
+     * We assume that policies that populate this form are loaded directly from ES and so
+     * are valid according to ES. There may be settings in the policy created through the ILM
+     * API that the UI does not cater for, like the unfollow action. We do not want to overwrite
+     * the configuration for these actions in the UI.
+     */
+    it('preserves policy settings it did not configure', async () => {
+      httpRequestsMockHelpers.setLoadPolicies([POLICY_WITH_KNOWN_AND_UNKNOWN_FIELDS]);
+      httpRequestsMockHelpers.setLoadSnapshotPolicies([]);
+      httpRequestsMockHelpers.setListNodes({
+        nodesByRoles: {},
+        nodesByAttributes: { test: ['123'] },
+        isUsingDeprecatedDataRoleConfig: false,
+      });
+
+      await act(async () => {
+        testBed = await setup();
+      });
+
+      const { component, actions } = testBed;
+      component.update();
+
+      // Set max docs to test whether we keep the unknown fields in that object after serializing
+      await actions.hot.setMaxDocs('1000');
+      // Remove the delete phase to ensure that we also correctly remove data
+      await actions.delete.enable(false);
+      await actions.savePolicy();
+
+      const latestRequest = server.requests[server.requests.length - 1];
+      const entirePolicy = JSON.parse(JSON.parse(latestRequest.requestBody).body);
+
+      expect(entirePolicy).toEqual({
+        foo: 'bar', // Made up value
+        name: 'my_policy',
+        phases: {
+          hot: {
+            actions: {
+              rollover: {
+                max_docs: 1000,
+                max_size: '50gb',
+                unknown_setting: 123, // Made up setting that should stay preserved
+              },
+              set_priority: {
+                priority: 100,
+              },
+            },
+            min_age: '0ms',
+          },
+          warm: {
+            actions: {
+              my_unfollow_action: {}, // Made up action
+              set_priority: {
+                priority: 22,
+                unknown_setting: true,
+              },
+            },
+            min_age: '0ms',
+          },
+        },
+      });
+    });
+  });
+
   describe('hot phase', () => {
     describe('serialization', () => {
       beforeEach(async () => {
diff --git a/x-pack/plugins/index_lifecycle_management/__jest__/components/edit_policy.test.tsx b/x-pack/plugins/index_lifecycle_management/__jest__/components/edit_policy.test.tsx
index 3e1577d8033ba7..eb17402a469501 100644
--- a/x-pack/plugins/index_lifecycle_management/__jest__/components/edit_policy.test.tsx
+++ b/x-pack/plugins/index_lifecycle_management/__jest__/components/edit_policy.test.tsx
@@ -298,12 +298,12 @@ describe('edit policy', () => {
             phases: {
               hot: {
                 actions: {
-                  set_priority: {
-                    priority: 100,
-                  },
                   rollover: {
-                    max_size: '50gb',
                     max_age: '30d',
+                    max_size: '50gb',
+                  },
+                  set_priority: {
+                    priority: 100,
                   },
                 },
                 min_age: '0ms',
diff --git a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/deserializer.ts b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/deserializer.ts
index 5af8807f2dec82..df5d6e2f80c15b 100644
--- a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/deserializer.ts
+++ b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/deserializer.ts
@@ -22,13 +22,11 @@ export const deserializer = (policy: SerializedPolicy): FormInternal => {
   const _meta: FormInternal['_meta'] = {
     hot: {
       useRollover: Boolean(hot?.actions?.rollover),
-      forceMergeEnabled: Boolean(hot?.actions?.forcemerge),
       bestCompression: hot?.actions?.forcemerge?.index_codec === 'best_compression',
     },
     warm: {
       enabled: Boolean(warm),
       warmPhaseOnRollover: Boolean(warm?.min_age === '0ms'),
-      forceMergeEnabled: Boolean(warm?.actions?.forcemerge),
       bestCompression: warm?.actions?.forcemerge?.index_codec === 'best_compression',
       dataTierAllocationType: determineDataTierAllocationType(warm?.actions),
     },
diff --git a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/deserializer_and_serializer.test.ts b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/deserializer_and_serializer.test.ts
new file mode 100644
index 00000000000000..b379cb3956a022
--- /dev/null
+++ b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/deserializer_and_serializer.test.ts
@@ -0,0 +1,201 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+import { setAutoFreeze } from 'immer';
+import { cloneDeep } from 'lodash';
+import { SerializedPolicy } from '../../../../../common/types';
+import { deserializer } from './deserializer';
+import { createSerializer } from './serializer';
+import { FormInternal } from '../types';
+
+const isObject = (v: unknown): v is { [key: string]: any } =>
+  Object.prototype.toString.call(v) === '[object Object]';
+
+const unknownValue = { some: 'value' };
+
+const populateWithUnknownEntries = (v: unknown) => {
+  if (isObject(v)) {
+    for (const key of Object.keys(v)) {
+      if (['require', 'include', 'exclude'].includes(key)) continue; // this will generate an invalid policy
+      populateWithUnknownEntries(v[key]);
+    }
+    v.unknown = unknownValue;
+    return;
+  }
+  if (Array.isArray(v)) {
+    v.forEach(populateWithUnknownEntries);
+  }
+};
+
+const originalPolicy: SerializedPolicy = {
+  name: 'test',
+  phases: {
+    hot: {
+      actions: {
+        rollover: {
+          max_age: '1d',
+          max_size: '10gb',
+          max_docs: 1000,
+        },
+        forcemerge: {
+          index_codec: 'best_compression',
+          max_num_segments: 22,
+        },
+        set_priority: {
+          priority: 1,
+        },
+      },
+      min_age: '12ms',
+    },
+    warm: {
+      min_age: '12ms',
+      actions: {
+        shrink: { number_of_shards: 12 },
+        allocate: {
+          number_of_replicas: 3,
+        },
+        set_priority: {
+          priority: 10,
+        },
+        migrate: { enabled: false },
+      },
+    },
+    cold: {
+      min_age: '30ms',
+      actions: {
+        allocate: {
+          number_of_replicas: 12,
+          require: { test: 'my_value' },
+          include: { test: 'my_value' },
+          exclude: { test: 'my_value' },
+        },
+        freeze: {},
+        set_priority: {
+          priority: 12,
+        },
+      },
+    },
+    delete: {
+      min_age: '33ms',
+      actions: {
+        delete: {
+          delete_searchable_snapshot: true,
+        },
+        wait_for_snapshot: {
+          policy: 'test',
+        },
+      },
+    },
+  },
+};
+
+describe('deserializer and serializer', () => {
+  let policy: SerializedPolicy;
+  let serializer: ReturnType<typeof createSerializer>;
+  let formInternal: FormInternal;
+
+  // So that we can modify produced form objects
+  beforeAll(() => setAutoFreeze(false));
+  // This is the default in dev, so change back to true (https://github.com/immerjs/immer/blob/master/docs/freezing.md)
+  afterAll(() => setAutoFreeze(true));
+
+  beforeEach(() => {
+    policy = cloneDeep(originalPolicy);
+    formInternal = deserializer(policy);
+    // Because the policy object is not deepCloned by the form lib we
+    // clone here so that we can mutate the policy and preserve the
+    // original reference in the createSerializer
+    serializer = createSerializer(cloneDeep(policy));
+  });
+
+  it('preserves any unknown policy settings', () => {
+    const thisTestPolicy = cloneDeep(originalPolicy);
+    // We populate all levels of the policy with entries our UI does not know about
+    populateWithUnknownEntries(thisTestPolicy);
+    serializer = createSerializer(thisTestPolicy);
+
+    const copyOfThisTestPolicy = cloneDeep(thisTestPolicy);
+
+    expect(serializer(deserializer(thisTestPolicy))).toEqual(thisTestPolicy);
+
+    // Assert that the policy we passed in is unaltered after deserialization and serialization
+    expect(thisTestPolicy).not.toBe(copyOfThisTestPolicy);
+    expect(thisTestPolicy).toEqual(copyOfThisTestPolicy);
+  });
+
+  it('removes all phases if they were disabled in the form', () => {
+    formInternal._meta.warm.enabled = false;
+    formInternal._meta.cold.enabled = false;
+    formInternal._meta.delete.enabled = false;
+
+    expect(serializer(formInternal)).toEqual({
+      name: 'test',
+      phases: {
+        hot: policy.phases.hot, // We expect to see only the hot phase
+      },
+    });
+  });
+
+  it('removes the forcemerge action if it is disabled in the form', () => {
+    delete formInternal.phases.hot!.actions.forcemerge;
+    delete formInternal.phases.warm!.actions.forcemerge;
+
+    const result = serializer(formInternal);
+
+    expect(result.phases.hot!.actions.forcemerge).toBeUndefined();
+    expect(result.phases.warm!.actions.forcemerge).toBeUndefined();
+  });
+
+  it('removes set priority if it is disabled in the form', () => {
+    delete formInternal.phases.hot!.actions.set_priority;
+    delete formInternal.phases.warm!.actions.set_priority;
+    delete formInternal.phases.cold!.actions.set_priority;
+
+    const result = serializer(formInternal);
+
+    expect(result.phases.hot!.actions.set_priority).toBeUndefined();
+    expect(result.phases.warm!.actions.set_priority).toBeUndefined();
+    expect(result.phases.cold!.actions.set_priority).toBeUndefined();
+  });
+
+  it('removes freeze setting in the cold phase if it is disabled in the form', () => {
+    formInternal._meta.cold.freezeEnabled = false;
+
+    const result = serializer(formInternal);
+
+    expect(result.phases.cold!.actions.freeze).toBeUndefined();
+  });
+
+  it('removes node attribute allocation when it is not selected in the form', () => {
+    // Change from 'node_attrs' to 'node_roles'
+    formInternal._meta.cold.dataTierAllocationType = 'node_roles';
+
+    const result = serializer(formInternal);
+
+    expect(result.phases.cold!.actions.allocate!.number_of_replicas).toBe(12);
+    expect(result.phases.cold!.actions.allocate!.require).toBeUndefined();
+    expect(result.phases.cold!.actions.allocate!.include).toBeUndefined();
+    expect(result.phases.cold!.actions.allocate!.exclude).toBeUndefined();
+  });
+
+  it('removes forcemerge and rollover config when rollover is disabled in hot phase', () => {
+    formInternal._meta.hot.useRollover = false;
+
+    const result = serializer(formInternal);
+
+    expect(result.phases.hot!.actions.rollover).toBeUndefined();
+    expect(result.phases.hot!.actions.forcemerge).toBeUndefined();
+  });
+
+  it('removes min_age from warm when rollover is enabled', () => {
+    formInternal._meta.hot.useRollover = true;
+    formInternal._meta.warm.warmPhaseOnRollover = true;
+
+    const result = serializer(formInternal);
+
+    expect(result.phases.warm!.min_age).toBeUndefined();
+  });
+});
diff --git a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/schema.ts b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/schema.ts
index 4d20db40187409..0ad2d923117f45 100644
--- a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/schema.ts
+++ b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/schema.ts
@@ -23,7 +23,7 @@ import { i18nTexts } from '../i18n_texts';
 const { emptyField, numberGreaterThanField } = fieldValidators;
 
 const serializers = {
-  stringToNumber: (v: string): any => (v ? parseInt(v, 10) : undefined),
+  stringToNumber: (v: string): any => (v != null ? parseInt(v, 10) : undefined),
 };
 
 export const schema: FormSchema<FormInternal> = {
diff --git a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer.ts b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer.ts
deleted file mode 100644
index 2274efda426ad1..00000000000000
--- a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer.ts
+++ /dev/null
@@ -1,185 +0,0 @@
-/*
- * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
- * or more contributor license agreements. Licensed under the Elastic License;
- * you may not use this file except in compliance with the Elastic License.
- */
-
-import { isEmpty, isNumber } from 'lodash';
-
-import { SerializedPolicy, SerializedActionWithAllocation } from '../../../../../common/types';
-
-import { FormInternal, DataAllocationMetaFields } from '../types';
-
-const serializeAllocateAction = (
-  { dataTierAllocationType, allocationNodeAttribute }: DataAllocationMetaFields,
-  newActions: SerializedActionWithAllocation = {},
-  originalActions: SerializedActionWithAllocation = {}
-): SerializedActionWithAllocation => {
-  const { allocate, migrate, ...rest } = newActions;
-  // First copy over all non-allocate and migrate actions.
-  const actions: SerializedActionWithAllocation = { allocate, migrate, ...rest };
-
-  switch (dataTierAllocationType) {
-    case 'node_attrs':
-      if (allocationNodeAttribute) {
-        const [name, value] = allocationNodeAttribute.split(':');
-        actions.allocate = {
-          // copy over any other allocate details like "number_of_replicas"
-          ...actions.allocate,
-          require: {
-            [name]: value,
-          },
-        };
-      } else {
-        // The form has been configured to use node attribute based allocation but no node attribute
-        // was selected. We fall back to what was originally selected in this case. This might be
-        // migrate.enabled: "false"
-        actions.migrate = originalActions.migrate;
-      }
-
-      // copy over the original include and exclude values until we can set them in the form.
-      if (!isEmpty(originalActions?.allocate?.include)) {
-        actions.allocate = {
-          ...actions.allocate,
-          include: { ...originalActions?.allocate?.include },
-        };
-      }
-
-      if (!isEmpty(originalActions?.allocate?.exclude)) {
-        actions.allocate = {
-          ...actions.allocate,
-          exclude: { ...originalActions?.allocate?.exclude },
-        };
-      }
-      break;
-    case 'none':
-      actions.migrate = { enabled: false };
-      break;
-    default:
-  }
-  return actions;
-};
-
-export const createSerializer = (originalPolicy?: SerializedPolicy) => (
-  data: FormInternal
-): SerializedPolicy => {
-  const { _meta, ...policy } = data;
-
-  if (!policy.phases || !policy.phases.hot) {
-    policy.phases = { hot: { actions: {} } };
-  }
-
-  /**
-   * HOT PHASE SERIALIZATION
-   */
-  if (policy.phases.hot) {
-    policy.phases.hot.min_age = originalPolicy?.phases.hot?.min_age ?? '0ms';
-  }
-
-  if (policy.phases.hot?.actions) {
-    if (policy.phases.hot.actions?.rollover && _meta.hot.useRollover) {
-      if (policy.phases.hot.actions.rollover.max_age) {
-        policy.phases.hot.actions.rollover.max_age = `${policy.phases.hot.actions.rollover.max_age}${_meta.hot.maxAgeUnit}`;
-      }
-
-      if (policy.phases.hot.actions.rollover.max_size) {
-        policy.phases.hot.actions.rollover.max_size = `${policy.phases.hot.actions.rollover.max_size}${_meta.hot.maxStorageSizeUnit}`;
-      }
-
-      if (_meta.hot.bestCompression && policy.phases.hot.actions?.forcemerge) {
-        policy.phases.hot.actions.forcemerge.index_codec = 'best_compression';
-      }
-    } else {
-      delete policy.phases.hot.actions?.rollover;
-    }
-  }
-
-  /**
-   * WARM PHASE SERIALIZATION
-   */
-  if (policy.phases.warm) {
-    // If warm phase on rollover is enabled, delete min age field
-    // An index lifecycle switches to warm phase when rollover occurs, so you cannot specify a warm phase time
-    // They are mutually exclusive
-    if (_meta.hot.useRollover && _meta.warm.warmPhaseOnRollover) {
-      delete policy.phases.warm.min_age;
-    } else if (
-      (!_meta.hot.useRollover || !_meta.warm.warmPhaseOnRollover) &&
-      policy.phases.warm.min_age
-    ) {
-      policy.phases.warm.min_age = `${policy.phases.warm.min_age}${_meta.warm.minAgeUnit}`;
-    }
-
-    policy.phases.warm.actions = serializeAllocateAction(
-      _meta.warm,
-      policy.phases.warm.actions,
-      originalPolicy?.phases.warm?.actions
-    );
-
-    if (
-      policy.phases.warm.actions.allocate &&
-      !policy.phases.warm.actions.allocate.require &&
-      !isNumber(policy.phases.warm.actions.allocate.number_of_replicas) &&
-      isEmpty(policy.phases.warm.actions.allocate.include) &&
-      isEmpty(policy.phases.warm.actions.allocate.exclude)
-    ) {
-      // remove allocate action if it does not define require or number of nodes
-      // and both include and exclude are empty objects (ES will fail to parse if we don't)
-      delete policy.phases.warm.actions.allocate;
-    }
-
-    if (_meta.warm.bestCompression && policy.phases.warm.actions?.forcemerge) {
-      policy.phases.warm.actions.forcemerge.index_codec = 'best_compression';
-    }
-  }
-
-  /**
-   * COLD PHASE SERIALIZATION
-   */
-  if (policy.phases.cold) {
-    if (policy.phases.cold.min_age) {
-      policy.phases.cold.min_age = `${policy.phases.cold.min_age}${_meta.cold.minAgeUnit}`;
-    }
-
-    policy.phases.cold.actions = serializeAllocateAction(
-      _meta.cold,
-      policy.phases.cold.actions,
-      originalPolicy?.phases.cold?.actions
-    );
-
-    if (
-      policy.phases.cold.actions.allocate &&
-      !policy.phases.cold.actions.allocate.require &&
-      !isNumber(policy.phases.cold.actions.allocate.number_of_replicas) &&
-      isEmpty(policy.phases.cold.actions.allocate.include) &&
-      isEmpty(policy.phases.cold.actions.allocate.exclude)
-    ) {
-      // remove allocate action if it does not define require or number of nodes
-      // and both include and exclude are empty objects (ES will fail to parse if we don't)
-      delete policy.phases.cold.actions.allocate;
-    }
-
-    if (_meta.cold.freezeEnabled) {
-      policy.phases.cold.actions.freeze = {};
-    }
-  }
-
-  /**
-   * DELETE PHASE SERIALIZATION
-   */
-  if (policy.phases.delete) {
-    if (policy.phases.delete.min_age) {
-      policy.phases.delete.min_age = `${policy.phases.delete.min_age}${_meta.delete.minAgeUnit}`;
-    }
-
-    if (originalPolicy?.phases.delete?.actions) {
-      const { wait_for_snapshot: __, ...rest } = originalPolicy.phases.delete.actions;
-      policy.phases.delete.actions = {
-        ...policy.phases.delete.actions,
-        ...rest,
-      };
-    }
-  }
-
-  return policy;
-};
diff --git a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer/index.ts b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer/index.ts
new file mode 100644
index 00000000000000..f901bfcf4d49d3
--- /dev/null
+++ b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer/index.ts
@@ -0,0 +1,7 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+export { createSerializer } from './serializer';
diff --git a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer/serialize_migrate_and_allocate_actions.ts b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer/serialize_migrate_and_allocate_actions.ts
new file mode 100644
index 00000000000000..d18a63d34c101e
--- /dev/null
+++ b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer/serialize_migrate_and_allocate_actions.ts
@@ -0,0 +1,73 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+import { isEmpty } from 'lodash';
+
+import { SerializedActionWithAllocation } from '../../../../../../common/types';
+
+import { DataAllocationMetaFields } from '../../types';
+
+export const serializeMigrateAndAllocateActions = (
+  { dataTierAllocationType, allocationNodeAttribute }: DataAllocationMetaFields,
+  newActions: SerializedActionWithAllocation = {},
+  originalActions: SerializedActionWithAllocation = {}
+): SerializedActionWithAllocation => {
+  const { allocate, migrate, ...otherActions } = newActions;
+
+  // First copy over all non-allocate and migrate actions.
+  const actions: SerializedActionWithAllocation = { ...otherActions };
+
+  // The UI only knows about include, exclude and require, so copy over all other values.
+  if (allocate) {
+    const { include, exclude, require, ...otherSettings } = allocate;
+    if (!isEmpty(otherSettings)) {
+      actions.allocate = { ...otherSettings };
+    }
+  }
+
+  switch (dataTierAllocationType) {
+    case 'node_attrs':
+      if (allocationNodeAttribute) {
+        const [name, value] = allocationNodeAttribute.split(':');
+        actions.allocate = {
+          // copy over any other allocate details like "number_of_replicas"
+          ...actions.allocate,
+          require: {
+            [name]: value,
+          },
+        };
+      } else {
+        // The form has been configured to use node attribute based allocation but no node attribute
+        // was selected. We fall back to what was originally selected in this case. This might be
+        // migrate.enabled: "false"
+        actions.migrate = originalActions.migrate;
+      }
+
+      // copy over the original include and exclude values until we can set them in the form.
+      if (!isEmpty(originalActions?.allocate?.include)) {
+        actions.allocate = {
+          ...actions.allocate,
+          include: { ...originalActions?.allocate?.include },
+        };
+      }
+
+      if (!isEmpty(originalActions?.allocate?.exclude)) {
+        actions.allocate = {
+          ...actions.allocate,
+          exclude: { ...originalActions?.allocate?.exclude },
+        };
+      }
+      break;
+    case 'none':
+      actions.migrate = {
+        ...originalActions?.migrate,
+        enabled: false,
+      };
+      break;
+    default:
+  }
+  return actions;
+};
diff --git a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer/serializer.ts b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer/serializer.ts
new file mode 100644
index 00000000000000..694f26abafe1d0
--- /dev/null
+++ b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/form/serializer/serializer.ts
@@ -0,0 +1,161 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+import { produce } from 'immer';
+
+import { merge } from 'lodash';
+
+import { SerializedPolicy } from '../../../../../../common/types';
+
+import { defaultPolicy } from '../../../../constants';
+
+import { FormInternal } from '../../types';
+
+import { serializeMigrateAndAllocateActions } from './serialize_migrate_and_allocate_actions';
+
+export const createSerializer = (originalPolicy?: SerializedPolicy) => (
+  data: FormInternal
+): SerializedPolicy => {
+  const { _meta, ...updatedPolicy } = data;
+
+  if (!updatedPolicy.phases || !updatedPolicy.phases.hot) {
+    updatedPolicy.phases = { hot: { actions: {} } };
+  }
+
+  return produce<SerializedPolicy>(originalPolicy ?? defaultPolicy, (draft) => {
+    // Copy over all updated fields
+    merge(draft, updatedPolicy);
+
+    // Next copy over all meta fields and delete any fields that have been removed
+    // by fields exposed in the form. It is very important that we do not delete
+    // data that the form does not control! E.g., unfollow action in hot phase.
+
+    /**
+     * HOT PHASE SERIALIZATION
+     */
+    if (draft.phases.hot) {
+      draft.phases.hot.min_age = draft.phases.hot.min_age ?? '0ms';
+    }
+
+    if (draft.phases.hot?.actions) {
+      const hotPhaseActions = draft.phases.hot.actions;
+      if (hotPhaseActions.rollover && _meta.hot.useRollover) {
+        if (hotPhaseActions.rollover.max_age) {
+          hotPhaseActions.rollover.max_age = `${hotPhaseActions.rollover.max_age}${_meta.hot.maxAgeUnit}`;
+        }
+
+        if (hotPhaseActions.rollover.max_size) {
+          hotPhaseActions.rollover.max_size = `${hotPhaseActions.rollover.max_size}${_meta.hot.maxStorageSizeUnit}`;
+        }
+
+        if (!updatedPolicy.phases.hot!.actions?.forcemerge) {
+          delete hotPhaseActions.forcemerge;
+        } else if (_meta.hot.bestCompression) {
+          hotPhaseActions.forcemerge!.index_codec = 'best_compression';
+        }
+
+        if (_meta.hot.bestCompression && hotPhaseActions.forcemerge) {
+          hotPhaseActions.forcemerge.index_codec = 'best_compression';
+        }
+      } else {
+        delete hotPhaseActions.rollover;
+        delete hotPhaseActions.forcemerge;
+      }
+
+      if (!updatedPolicy.phases.hot!.actions?.set_priority) {
+        delete hotPhaseActions.set_priority;
+      }
+    }
+
+    /**
+     * WARM PHASE SERIALIZATION
+     */
+    if (_meta.warm.enabled) {
+      const warmPhase = draft.phases.warm!;
+      // If warm phase on rollover is enabled, delete min age field
+      // An index lifecycle switches to warm phase when rollover occurs, so you cannot specify a warm phase time
+      // They are mutually exclusive
+      if (
+        (!_meta.hot.useRollover || !_meta.warm.warmPhaseOnRollover) &&
+        updatedPolicy.phases.warm!.min_age
+      ) {
+        warmPhase.min_age = `${updatedPolicy.phases.warm!.min_age}${_meta.warm.minAgeUnit}`;
+      } else {
+        delete warmPhase.min_age;
+      }
+
+      warmPhase.actions = serializeMigrateAndAllocateActions(
+        _meta.warm,
+        warmPhase.actions,
+        originalPolicy?.phases.warm?.actions
+      );
+
+      if (!updatedPolicy.phases.warm!.actions?.forcemerge) {
+        delete warmPhase.actions.forcemerge;
+      } else if (_meta.warm.bestCompression) {
+        warmPhase.actions.forcemerge!.index_codec = 'best_compression';
+      }
+
+      if (!updatedPolicy.phases.warm!.actions?.set_priority) {
+        delete warmPhase.actions.set_priority;
+      }
+
+      if (!updatedPolicy.phases.warm!.actions?.shrink) {
+        delete warmPhase.actions.shrink;
+      }
+    } else {
+      delete draft.phases.warm;
+    }
+
+    /**
+     * COLD PHASE SERIALIZATION
+     */
+    if (_meta.cold.enabled) {
+      const coldPhase = draft.phases.cold!;
+
+      if (updatedPolicy.phases.cold!.min_age) {
+        coldPhase.min_age = `${updatedPolicy.phases.cold!.min_age}${_meta.cold.minAgeUnit}`;
+      }
+
+      coldPhase.actions = serializeMigrateAndAllocateActions(
+        _meta.cold,
+        coldPhase.actions,
+        originalPolicy?.phases.cold?.actions
+      );
+
+      if (_meta.cold.freezeEnabled) {
+        coldPhase.actions.freeze = coldPhase.actions.freeze ?? {};
+      } else {
+        delete coldPhase.actions.freeze;
+      }
+
+      if (!updatedPolicy.phases.cold!.actions?.set_priority) {
+        delete coldPhase.actions.set_priority;
+      }
+    } else {
+      delete draft.phases.cold;
+    }
+
+    /**
+     * DELETE PHASE SERIALIZATION
+     */
+    if (_meta.delete.enabled) {
+      const deletePhase = draft.phases.delete!;
+      if (updatedPolicy.phases.delete!.min_age) {
+        deletePhase.min_age = `${updatedPolicy.phases.delete!.min_age}${_meta.delete.minAgeUnit}`;
+      }
+
+      if (
+        !updatedPolicy.phases.delete!.actions?.wait_for_snapshot &&
+        deletePhase.actions.wait_for_snapshot
+      ) {
+        delete deletePhase.actions.wait_for_snapshot;
+      }
+    } else {
+      delete draft.phases.delete;
+    }
+  });
+};
diff --git a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/types.ts b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/types.ts
index dc3d8a640e682d..7d512936290af0 100644
--- a/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/types.ts
+++ b/x-pack/plugins/index_lifecycle_management/public/application/sections/edit_policy/types.ts
@@ -18,7 +18,6 @@ export interface MinAgeField {
 }
 
 export interface ForcemergeFields {
-  forceMergeEnabled: boolean;
   bestCompression: boolean;
 }