It depends how you define "Impossible" - I can't see anywhere there where we have said it's impossible, or have mentioned BT?
In reality what we are talking about is cancelling one account and opening a brand new one and then copying over all of the details. As none of our support staff have access to things like payment details, we have to involve the finance team to transfer over payment details and there are other complexities (Eg people that want to keep fax2email numbers, static IPs etc, all of which are difficult to move).
I agree with Rsharma about what sounds like the right approach to this one - However to be able to do this requires a lot of development work to get us there. Use of usernames is engrained in a number of internal systems, and there is some quite hefty code rewrites needed to change that (Although for every system we have built in recent years, a unique service identifier has been used instead so it should be easier now than it would have been a year or two ago).
That said, I'm hoping we can come up with something a bit simpler that will work for everyone. I also agree that the postmaster requirement is a bit legacy now. We nabbed that requirement from Demon 10 years ago if I remember correctly and it's never been reconsidered, even though things are rather different now and most people do have an alternative address.
Ian