A word of warning... if there is a chance that you will have to cancel one room or modify your plans, you may be looking at a mess. I booked three rooms on the old AP system, the ones you could book by phone. Originally, we had six adults and two children. Two adults had a change of plans due to medical problems and one adult was leaving early. So, we went from three rooms for six nights to two rooms for six nights and one room for two nights. I called Disney Reservations TWO TIMES to make sure we knew how this should be handled. Both times, I was told that everything was fine and the change would be handled at check in. (I even had the name of a higher-up from Guest Services who OK'd the plan.) So, we arrive at the hotel, told the lady at the front desk what I'd been told, and she basically told me that the person leaving early would have to pay for all six days, sorry-bout-yer-luck. When I mentioned that I had the name of a Guest Service person who OK'd the plan, she said without even looking at the name (and I quote) "It doesn't matter. I don't know them." We went around for about an hour trying to work it out. (My state defense attorney brother was about one inch from totally loosing it.) We finally just gave up and went to our rooms. About 10 minutes later, my room phone rang-- it was the resort manager (who we hadn't even asked for), who was very VERY sympathetic and apologized profusely. He asked us to come back to the front desk and after a bit, had the whole thing straightened out. Nobody had to pay any extra, thank goodness. It was, as I told the manager, how it should have been from the beginning.
So, the moral of the story is, if you need to make any changes, it helps if you completely cancel your vacation and start over. Trying to modify an existing reservation using the AP rate is a total disastser.
Other than the bumpy start, we had a fabulous vacation! Disney Magical Express worked like a dream, we were very impressed with them... the changed reservation didn't even phase them!