@trwnh you are right, semantically a Move is the wrong thing here. The actor does not move anywhere, only its id changes, and a rewrite describes that much better. I tried to reflect that in a new draft: Annotate file.
There is no Move anymore. The publisher sends a normal Update, and a receiver detects the change when the alias resolves to a different id. Your trwnh / a_trwnh case is not affected by that, and nothing conflicts with FEP-7628 anymore.
The same-origin rule is only a scope limit, not part of the verification. You are right that the check itself works across origins, I just did not want to touch migrations.
Stored data does not need a rewrite. The alias keeps resolving and is listed in alsoKnownAs, so a stored Follow of the alias is still a follow of the actor. Only the actor record has to learn the new id. I also added HTTP examples for both variants, redirect and serving the document at the alias.