The problem with that is coordination. All the Mastodon servers would need to align, agree, and implement this new process. The whole point of Mastodon is that it is federated, and so consensus will ensure progress is limited to the lowest common denominator.

Federation works when protocols and standards are negotiated upon, and people do coordinate implicitly wherever they apply the protocol. So I'm not sure if there's anything to explain as if we're at cross purposes.

Why couldn't a protocol and reference implementation exist for a client migration if servers have similar schemas for users and posts?