This might actually be a great service for fastmail et al to provide: after you import old mailboxes, they should scan them for signup mails and show you a list with to migrate accounts i.e. red marks.
After migrating an account, they know it, as they can scan your mail. Slowly, each red mark on your dashboard gets replaced with a green mark.
Actually I was thinking it would be a good idea for a password manager to implement. They have all of the credential information, they'd just have to implement "Change my userid" logic for all websites out there. That's definitely a hard problem to solve...
After migrating an account, they know it, as they can scan your mail. Slowly, each red mark on your dashboard gets replaced with a green mark.