Like usual, the Mastodon people have the least understanding of how federation actually works.

When a server stops federating, the federated posts still exist in the DB of all the remote instances that recieved it. They're still indexed and searchable from there. You could move those posts into the DB of a new server, but they'll still "originate" from the old one. But you can carry on and interact with old threads.

There's no real foolproof mechanism for updating posts, although there is an AP Update activity, much like deletes there's nothing forcing an instance to respect it.