I submitted my proposal to FEP repository: fep/fep-7628.md at main - fep - Codeberg.org
And there’s now a dedicated topic on SocialHub: FEP-7628: Move actor
The proposal only covers followers, because migration of followers requires coordination between servers (using Move activity). Following, lists, mutes, blocks and bookmarks can be simply exported and then imported to a new server, so I think these are out of scope.
Overall, I’m beginning to think that this approach to migration is a technological dead end. It would be better if we stop attaching accounts and posts to any particular server.