Usually, but not always, or it may not be up to Disney.
We had booked parsailing and on the day of, my oldest daughter wanted to back out. We asked at the Cruise Desk and were told we would have to take it up with the excursion operator, and they MIGHT offer us a credit on a different excursion.
In the end she (wo)manned up and did it and ended up loving it
-dabe