You can call the dining reservations line and tell the castmember you now have the dining plan. Be sure the castmember understands you want to keep the reservation and just change the payment method. It's easier for the castmember to cancel the reservation and refund your credit card, but in that time someone else might pick up your reservation and then you would be out of luck. You will still have to give your credit card information for the no show/didn't cancel guarantee which for HDD is the full cost of the meal.
Alternatively, you could wait until you are at DW and when you pick up your HDD tickets you can have your credit card refunded at that time.