exebat And what would be the harder way ?
Don’t know, but changing the domain of already existing mailboxes doesn’t sound like something that is really supported or even possible. Maybe there are some hacky ways to do it, which probably should be avoided… ;-)
If you don’t want to use a Domain Alias, I would create new mail boxes on domain.com and then migrate the mails over to the new mail boxes using IMAP Sync. Basically the same thing again you just did in order to move them over from Gmail.