It should be noted that, at least for now, account migration can only go one way. That is, from the main Bluesky PDS to an external one. According to the documentation on account migration[0], they plan to enable incoming migrations at some point in the future, but there's no timeline.

I'm sure a big reason for this the possibility for abuse, since "migrating" an account is a different action than simply "creating" a new one

[0]: https://github.com/bluesky-social/pds/blob/main/ACCOUNT_MIGR...