Just did this with a buddy on VZW. The store shut him down and the insurance company shut him down. But then we just picked up the phone and took turns calling service reps at VZW till we found one that actually offered to authorize it and then call the store to make sure it went through well.
He went from a 5 year old smartphone (older than a treo 650) to last years samsung i760.
Not bad! So its possible.
