Even if the username changed, having some feature where you can transfer your account and follows, history and notify all to your followers of the new name so their follow list gets updated could be coded, and it would make instance hopping relatively easy.

I'm against reserving your name in all instances but having a feature to change the handle and that all your followers don't suddenly know who you are is important. Something akin to creating the new one, notifying of the change and leaving the old one directing to the new one for some time.

Or just not letting you swap to an instance if the handle exists. Idk. But this would make me actually start using mastodon if it were a feature. I might even start a fork and work on it.