If you haven't gotten your tickets yet, you can call and have them changed and pay the difference. Some people here have done it. Once they are issued, they are not upgradeable. We are leaving next Thursday and I plan to add water parks/DQ to the 2 tickets we didn't originally had that on. I know someone posted the phone number, so I'll go look.
So, the number is 407-472-4676. I called and Dave answered the phone. He was able to upgrade my hoppers to premium tickets. He said sometimes it is not possible to make changes closer to your arrival date, but we are arriving July 6th and he was able to do mine. BTW, I told him that he has a reputation for good customer service over on the Disboards and he was very humble that he's just doing his job. Gotta love a guy like that!