Martin Man (the original developer) and myself spent over three months exchanging email and phone calls with Apple in the hope that we could find a way to retain the existing users. Apple's response to us was completely consistent: they told us that ownership transfer is only possible if we stop selling the original app from the developer's account and then begin selling it from our own business account.
If you search the web you will see that almost every other vendor in our position has been told the same thing; however there are some rare cases where people claim to have gained an exception so that existing users and reviews (we lost 2 years of reviews in this process) could be retained. I presume this is what happened with Twittee, but I have no way to verify this. All I can tell you is that after three months of trying, our only option was to comply with Apple's requirements.