@evan @luis_in_brief @simon previously https://github.com/mastodon/mastodon/issues/177#issuecomment-292780728
We had a mechanism for this in OStatus that worked by delegating to the http and dns stacks. AP half bought into the dns is bad, keys are good worldview so to migrate you need a separate protocol to do a key handover and follower migration to the new instance. However historic posts are stuck on the old instance because it can't 301 them to the new one because signing posts is bound to the user, not the domain.