Yes, it's a sure thing. I have done it before. The prepurchase you made says this
I did this several years back when I was worried that for some reason even though I went to DLR last (we also an AP) I would still get charged the WDW price. So I waited until after I ordered my CD (paying $59.95) before I asked for a refund.
Since the pre-buy has now expiration on it (except for the dates of your photos) you could use it at any time and therefore you can request a refund at any time. What if you went to the parks and didn't get many PP taken. Or you didn't like enough of them? That's why they offer the money back guarantee.
We'll be at WDW in about a week as well. I will be going to DLR after we get back (probably several trips) to get more PP taken. That will set the pricing to the DLR one. I will use the coupon code for the Gallery CD I am buying tomorrow to get the price to zero. Trust me, it works. I've used the coupon code 3 times so far. Twice it inc. photos from WDW. Once they were mine, once they were my DSis (this past May).
I don't know how many Gallery CDs they have in stock but I am getting mine tomorrow, just in case they run out again. They ran out this year for a few months and I don't want to risk it again. If you can squeeze in a visit to DLR you might want to pick one up.
BTW, I would probably go ahead and enter your PP cards into a new PP account. Use a different email address. Then when you ask for the refund there won't even be any photos in your account.