Well, then maybe it's not ... I only meant that, either way, the mail gets delivered as you desire. Sorry for not being clearer about what I meant - if your aliases both work you won't "lose" mail (incoming mail will be delivered) Now, preserving exisiting IMAP mail is a different thing altogether, and I mentioned that later on
I see, so what you need to do is migrate the mail from one account to the other, right?
Assuming that both email accounts are actual Mailboxes associated with machine users (with Maildirs you can reach from the shell), as opposed to "email only users", there are a several approaches you could take besides uploading/downloading (which I only mentioned in relation to POP3 - and that is what you would have to do in that case, as there is no mail left on the server to manipulate).
Some of them are discussed here:
Is that what you are looking for?