Unless you had to put a credit card down for booking, you won't be charged if you don't show up.
Some ADRs (i.e. dinner shows and CRT) require full payment at booking, and if you don't cancel before the official cancellation time, you're out the full price. QUOTE]
We had that happen at Spirit of Aloha dinner show. We had prepaid, and tried to cancel our reservation the night before. I think they have a 48-hour cancellation policy - because we called only 24 hours in advance to cancel, they wouldn't refund our money. We ended up going to the show, since we had already paid (and enjoyed it). But with regular restaurants where you don't have to give a credit card and pre-pay (like San Angel), you won't be charged. If you're lucky enough to get in at Le Cellier, just call to cancel your other reservation as a courtesy.
Hate to be a wet blanket, but you probably won't be able to get a walk-up at Le Cellier, though - it's the most popular restaurant at Epcot and very hard to get into (as you have already discovered). We were able to get 9:00 dinner reservations one day, by going to the restaurant right when they opened that morning - but by the time we got our food, it was about 9:45 (way too late for dinner for me - I don't mind eating later in the evening, but that's too late!).