That would be very nice, though how it would be implemented would be difficult. One way it could potentially be done is the new server has to request control from the old one, that would be fairly secure but would introduce the problem of “What if the old one no longer exists?” It could be that there’s a key that you send with your activity to verify it’s you which can be moved to a new instance, but then you run into the issue of how to stop a malicious instance admin from getting your info. You could have a centralized authentication service, though many dislike that because it goes against federation to some extent. I know on Mastadon you can transition your followers, I don’t know how it works but it’s possible they figured out a good solution.