Last year I had a reservation that I had to hold with a cc. Probably 18 hours before the reservation it looked like it would be really out of the way to go to that restaurant due to scheduling changes. I talked to the conceirge and she found me a few other places I could eat. I asked her to cancel my cc held reservation and take a different one. She looked at me with big eyes and said, "You don't want to cancel your reservation, that will get you charged. You want to modify your reservation. Never say cancel!" So I modified my reservation- to a different time and a different restaurant. No charges! I suppose that things could come up where you just plain have to cancel but if the issue is just that you want to stay in a different park, or you want to go back to the hotel and not travel back to the park, etc. ask if you can modify your reservation.
On a different trip we thought my son had fractured his arm. We took him to the first aid station to get directions to an urgent care. While there I said, "Ahh... we have reservations. If it's broken we won't make it back in time!" The CM at the first aid station told me not to worry about it, got the reservation info from me and after we left contacted the restaurant to take care of everything. I know they did what they said because (Thank the Lord!) his arm wasn't broken and we made it back in time. When we showed up for our meal the person checking us in actually asked about his arm and was glad that we were able to finish our day!
My point with these is that sometimes things do work out well, even when you think it can't possibly!