I filed a bug for #ActivityPub, which is exactly about Person's freedom to choose (and actually, change) an instance/server of a global social network without loosing his/her identity, including historical data.
Moving from one instance of a federation to another is a normal case, just like having user accounts at several servers. And #ActivityPub specification should explicitly allow this.
See https://github.com/w3c/activitypub/issues/260