Alright it's late and i need to go to bed, but here's a draft FEP to do full account migration with posts and whatever other kinda objects you want to bring with you. It's a trivial expansion of existing ActivityPub/streams systems and supports gradual migration as it's implemented and after an account migration. It should be possible to migrate pretty much everything this way, both private and public objects.
p3x.de
jonny (good kind)