As everyone has mentioned, the only ones with penalties is the one that require a credit card. When I called to change ressies, they were able to give me a list of my ressies and found some ressies (that I had originally cancelled, but was not done) and asked me which one I would like to keep. I don't think they like the idea of having more than one ressie at similar times. I had two ressies in one day for dinner and one I cancelled and the other one I didn't even make...so I have no idea what happened there.
Anyways, I am one of those who cancel ressies as soon as I know for sure that I will not be using it so it can be given to others. I would hope the same courtesy.
I understand that it's difficult to know where you will be and that's understandable, but there's quite a few people desperately trying to get ressies. When I cancelled my Coral Reef ressie to change it to another day, the CM had told me how much someone would appreciate this. I suppose she mentioned it because it was sold out for that day.
By the way, if you know where you will be, you can still cancel it at guest relations and they will promptly put it back in the system and hopefully someone can make use of it. I had gone to change one of my ressies for the next day at MK and it so happen that the lady next to me had been wanting a reservation for that same location and time and as soon as CM cancelled it the CM next to me said to the woman (she was somewhat distraught) that a ressie has just come up and she could have it. Believe me, she was very very happy. The CM assisting me had overheard and mentioned it was me, (although, I didn't do much) she came over and thanked me. But I told her that she didn't have to thank me, I don't really think I did much. But she had said that some people don't take the time to cancel so the ressie is unused. And it was nice to see someone do it.
Both CMs agreed.
