Minor adjustments to the documentation for the JSON schema version

README.md

@@ -49,7 +49,7 @@ changed, we will also update the schema version number. The version number is
 represented as a single integer. Any changes to the schema are assumed to be
 backwards-incompatible with previous versions of the schema, so all version
 bumps should be treated as major version bumps. Each change to the schema is
-described in the [`SCHEMA_VERSIONING.md`](SCHEMA_VERSIONING.md) file.
+described in the [`SCHEMA_CHANGELOG.md`](SCHEMA_CHANGELOG.md) file.
 
 
 [crux-mir-repo]: https://github.com/GaloisInc/crucible/tree/master/crux-mir

SCHEMA_CHANGELOG.md

@@ -5,4 +5,13 @@ versioned separately.)
 
 ## 1
 
-Initial schema version.
+Initial schema version, as of 20240910.
+
+## Unversioned files prior to version 1
+
+The schema used between 20240823 and 20240910 is the same as version
+1, except without the version tag. Output from before this point is
+different in various ways and no longer supported. In general any
+unversioned files one may encounter should be recompiled; however,
+any that came from a version of mir-json before 20240823 will not
+be readable by downstream tools and must be rebuilt.