How do I move an existing custom tier sponsor to a new non-custom tier with the same donation amount?

[TomerAberbach]

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I have a sponsor who sponsored me for $10 per month via a custom tier. Today I added a new non-custom tier that's also $10 per month and grants access to a private repository.

However, the existing custom tier sponsor is still associated with a custom tier in my dashboard, so they don't have access to the private repository. The dropdown shows "$10 a month" twice...

How do I move this existing sponsor to the new non-custom tier?