I was told that yes you could, but, the chances of you getting the lower rate were slim because apparently they block out a number of rooms at the discounted price. But, then again, she could have been wanting me to book at the higher price.
I booked through Costco because of the free Character meal and the gift card. The amount of the gift card changed from when I booked to now and I called and got the higher amount.
We've always booked through Costco and it is very easy to make changes. I have never had an issue getting whatever the new rate is. I check a few times a week while paying off our vacation and call immediately if I see a new offer.
We booked with Costco too. I just got off the phone with them because I noticed that they are having a promotion that will allow you to stay for longer than the 4 nights that they originally told we could only book for, and on top of that, instead of the $20 Disney gift card, we are getting $60 for booking 5 nights! Yay!! Score!!
I actually just asked that exact question. If it is more than 30 days out you can make any changes you want to your reservation without fees. If it is less than 30 days out you will have to pay a $50 change fee for any changes in reservations or cancelations (this does include price drops).
They did tell me that just because their is a price drop it doesn't mean that you will be able to book the room at the lower price due to limited availability. However, you can always cancel your room if you can get it for a lower price elsewhere.