It should be the last resort plan. Cuz if the domain name changes, that's a new instance as far as Lemmy is concerned. Sure, the instance will hold all your posts and media, but I don't think new posts made from that account will be associated with posts before the domain name change.
Lemmy.ml is the instance ran by the developers of the Lemmy software. I’m sure they would just love to some sort of key verification for federation than just domain name, which they should be doing anyway because domain names can change.