How late is the park open?
I've heard of people getting later times closer to their date. But, I think they stop serving maybe an hour or so before closing, not sure on the exact time.
DH and I tried to eat there before Fantasmic one night, we had a good 2 hours before it started, they weren't taking anyone else for dinner.
I would just take what you can get for right now. Try later and see if you can change it. At least you'll have a dining option. If you decide you want to cancel it, no big deal.