I'm not 100% on this but I'm guessing there has to be a 24-hour or maybe even 48-hour cancellation policy. I tried to look it up on the official site and didn't see anything... I would contact Disney directly to ask though.
When we booked, we booked for 3 people. When the day came, one person couldn't make it. So we went to the concierge at our hotel and asked what we should do. We were not expecting any money or anything, just wanted to let them know that we had an extra spot for someone.
They refunded us our money and our reservation of 3 became a reservation of 2. We were quite surprised.
Your best option would be to call disney and find out your best option. If you go to the thread in my signature, there is a phone number you can call on the first picture of the thread.
Hope all works out for you!