Consider the common analogy of e-mail: your e-mail account is defined by its address, because that’s the key piece of info required to do things with that account, like send e-mail from it, or to it. If you change the name of the server that your e-mail account belongs to (E.G. @mymail.com to @mymail2.com) that, by definition, is a different account. Lemmy user accounts work effectively the same way.
Of course, with e-mail, it’s not terribly difficult to just take the history of all your e-mails from the old server, and move them, but those e-mails are all still gonna show as “from” or “to” the old address, unless you also go through and re-write the history. Lemmy currently does not have any such “migration” procedure, and also suffers from a variety of design constraints that would make this much more difficult than our theoretical e-mail history migration, most notably the problem of how to synchronizd this mgration action across all instances.
This issue has actually already come up in practice, BTW. The instance VLemmy basically vanished off the internet a month or so ago, apparently due to loss of their domain name rights, right in the middle of the reddit migration. There were a lot of people asking whether the whole INSTANCE could be migrated to a new domain name. If that wasn’t enough push to get migration capabilities designed and added, I doubt anything will.