It's possible, it just takes some time. The same thing happened to me. I needed a new phone before I went off to school, so my dad agreed to let me take his upgrade. When we got the phone in the mail, we took it to the AT&T store and explained what we wanted to do, and the rep was more than willing to help us swap sims, activate the phone, etc. in order to set up the i4 under my number. The result: I had a shiny new phone to take school and my dad was able to keep his old phone and number until my line was eligible for upgrade.