Once you add the dining plan to your resort reservation you can call the dining line and have your method of payment modified. Be sure the CM understands that you do not want to cancel the reservation, just change the method of payment. Cancelling the reservation is the easiest way to refund your credit card, but you do not want this to happen because someone else might snap up the reservation in the brief time after you've cancelled and then you'll be without a res. If the CM wants to do this you can either either ask for a supervisor or hang up and try again.
The second option is to wait until you arrive at CRT and let your server know that you want to use dining credits and s/he will refund your cc.