Since you'll be at HDD next week, I would wait until you pick up the tickets and at that time let the CM know you want to use dining plan credits. The credit card you used when you made the HDD reservation will be credited back. Any credit we've gotten has been posted by Disney within a day or two; after that it depends on how long it takes your bank/cc company.
You can call the dining line and have it credited back sooner, but some of the dining line CMs are not familiar with the process and will cancel your HDD reservation entirely since that's the easiest way to get a credit back to your card. Unfortunately, in the time between cancelling and looking for a new reservation someone else could be looking and grab the reservation you just cancelled. Then you'll be without a reservation and another one for Dec 26 could be very hard to find. If you want your credit immediately, make sure the CM on the phone knows you're just modifying the payment method and be very clear that you do not want to cancel the reservation. If the CM wants to cancel the res, ask for a supervisor or hang up and call again.