Technically, changing usernames can be done on various Fediverse platforms following Mastodon’s protocol for moving to a different domain. There’s no reason why the source and destination domain would need to be the same.

Practically, usernames are used as unique identifiers in most ActivityPub and wider Fediverse implementations, making changing usernames quite difficult.

When you follow an account, you follow a username. This is looked up using a protocol called “WebFinger”, named after the old UNIX “finger” command. This could in theory return something akin to a user ID rather than a link to a document with a user name, but I haven’t seen many implementations of that.