Hi Barbara,
How exciting that you received Free Dining and can get a refund now!
You have 3 options in this situation. First keep in mind that the dining credits
can only be used for Category 2 & 3 seating for this show.
1) You can call 407-WDW-DINE and see if they can re-book the
Hoop-Dee-Doo Musical Revue using your Disney Dining Plan credits and then cancel your original reservation for a refund.
2) If there isn't anything available in the same category that you wanted, then the agent can contact the Dining Help department for assistance with changing your existing reservation. This is a bit of process, but it can be done. Plan on spending about 15 minutes on hold while they take care of this. You'll still need a credit card guarantee to hold the booking, even when using the Dining Plan. However, the card will not be charged and a hold won't be put on it. This is simply a guarantee and you would only be charged if you don't show and don't cancel.
3) If you just want to keep your existing reservation, you can
bring the same credit card that you paid with to the show with you. When you check in at the podium, you'll simply tell them that you didn't have the Disney Dining Plan when you booked and paid in advance. Tell them you'd like to use it now and present them with your Key to the World Card. They will subtract 2 credits per person for the show and will refund the amount originally paid to your credit card.
If you don't need the money right now, I'd choose option 3 so I wouldn't have to worry about losing my spot. However, I have personally been in this same situation (at Cinderella's Royal Table instead) and I wanted the credit on my card for my trip. I was able to do option 2 and they refunded my money that same day.
Hope this helps and have a wonderful vacation,
Kaylene