When I got my first AP I rolled my ticket package in to an AP, you just pay the different between what you paid for the tickets and the AP cost, and it was super easy. My DH has never had an AP, and with at least 2 trips after our next scheduled and in the works we've decided to do the same for his. You just have to use your ticket once and then upgrade before you use your last (I believe, I know you have to use a ticket first). I actually did it again this year when my AP lapsed and both times I went in to the park on my first day and then stopped by Guest Relations on my way out to upgrade; the first time I did it at the guest relations outside of MK, the second time I did it at guest relations inside the park. I like to do it this way so I can start using my 20% merch discount, and 10% off TS.
You could always call and see if you can get a room package with just the room and dining plan and purchase an AP voucher if you are sure you want an AP, or just do it the easy way and upgrade when you get there. There is no guarantee there will be any good discounts for a while so I wouldn't bank on there being an AP discount since those aren't guaranteed and only used to fill rooms. Either way it costs you the same money, it's just kind of like you've pre-paid for some of the AP ahead of time, they already did a price increase right before the shut down so you won't be in danger of another one any time soon and you'll likely come out paying the same.
