Honestly the best way to get an answer on this is to ask a rep at Best Buy what your options are. I have come to realize that responses such as "I would suspect..." or "whenever AT&T has deals like this..." are only speculation at best. Rules and regulations on these matters change all the time.
For what it's worth, my wife purchased a phone on a monthly plan from Best Buy a couple of years ago and yes they let her pay the phone off after having signing up and paying the down payment. However, when I tried this a year later, they said I could not do this.
Just ask them when you are ready.