Our current thinking is that we aren't going to differentiate between users who have already migrated another device and those that haven't. (Note that we might try and detect if the user has devices that *aren't* yet migrated so we can say either "and you are all done" vs "you need to migrate other devices too". So the "Join the party" box probably needs to be removed, and the box that says "is legacy username an email address" will say something like "if the user has previously migrated, get the account name" and "if the user hasn't previously migrated, see if the legacy account name is an email address", both of which feed into the "Autofill email..." box.

Current UX flows now capture this.