The easiest method to upgrade from a different line is to do it at an AT&T corporate store. I've done this with other phones in my family plan, as my children are always wanting to borrow the other child's upgrade status for the "latest and greatest" phones.
If you do it at a corporate store, they can do all of the computer work, switching of lines, etc. without you having to do a thing except sit there and scan your credit card when it is all over.
They do it all the time, and I've never had a problem. Good luck. I'm ready for the new one myself and so is my son. He has been saving up for a long time for this, he has my old original phone. I have a 3G and we are both eligible for an upgrade right now.