Considering the system has a way to federate a server self destructing telling other servers to delete its posts could a similar system not just tell other servers to update their URL for that users posts.

Not pretending to know anything about the system feel free to point out why this wouldn't work. I'm just approaching it from a user perspective that having to lose posts to move server is bad for users and having to choose to nuke all posts in the case of shutting down a server is bad for admins.