Migrate existing Sign in with Apple user identifiers and private email relay addresses by exchanging transfer identifiers from one developer team to another with the user migration info endpoint.
TN3519: Migrating Sign in with Apple users for an app transfer
This technote is helpful. What if I don't have the sub values (unique identifiers) of Apple Sign in users? Is there a way to get them retroactively? Also, if I don't migrate the Apple Sign in users, will the privaterelay emails change for the new team? That's what I use to authenticate and track app progress.