Hi there, William!
This is a very common occurrence, and the Cast Members at
Cinderella's Royal Table will be able to accommodate you with ease.
You are welcome to call and ask for a refund; however, this might mean you'll have to release and rebook your reservation. That's a risk you might not want to take.
The more "secure" way to go about this is as follows: When you arrive at the restaurant, just inform the maitre d' and your server that you are on the dining plan and need a refund. Have the appropriate credit card with you.
The Cast Members will take the credit card and apply a refund to it; you'll subsequently be charged the right number of credits per person at the completion of your meal. These credits will be debited from your Keys to the World.
Have a wonderful time!
See you later, celebrator,
-Deborah B.