I really don't think that they will give you a discount. We were going to be arriving late, so I called the resort to let them know. They said it was okay because they actually go ahead and assign your room to you when you make the reservation. That being said, they are "holding" your room for you meaning they couldn't have given it to someone else. But...I guess it won't hurt to ask the Disney reservationist when you call.
Hope you get what you're looking for, Lisa