Since you can now have the
DDP with an AP this may have changed, but...
when we went last December I had planned on changing our tickets to APs. First, you absolutely have to do it at Guest Relations at one of the parks. We tried to upgrade at check in and with guest relations in the hotel. Second, and most importantly for us, they told us if we upgraded at the beginning of our stay we would lose the DDP we had attached to our MYW tickets. We had the girl double check with someone in the back to be sure and they said the same thing. We could upgrade on our last day in the parks however (on our way out) and get full credit for what we had spent on the tickets (something about 14 days they keep their initial value). Well, we (both parents and both toddlers) got violently ill with a stomach flu and we left early without even going back to the parks the last day. It was then XMas right after we got well and I forgot all about it until right after New Years when it was too late to even call and upgrade. So sad. We lost a lot of money on that trip between not getting the AP tickets and not using half our TS and CS credits, as well as none of our snack credits. We had planned to go to DTD to use our snack credits on our way out of town. As it was, we were lucky to be able to drive home we were so sick!
Anyway, all that rambling was just to say it may cause a problem with your DDP. Just so you are aware. Maybe you can email and find out before you go so you don't waste any time at guest relations if you can't. Good luck!