Hi Tammie,
If you did decide to add the Disney Dining Plan once you have already made advance dining reservations at dining locations which required the full amount to be pre-paid at the time of reservation, you would have two options:-
1/ When you arrive at the restaurant(s) for your meal, simply explain to your server that you prepaid for your meal on your credit card when making your dining reservation but would like to use your Dining Plan credits instead.
They will deduct table-service meal credits from your Dining Plan for each Guest in your party (Aged 3 and over) and will refund your credit card there and then. (You will however need to take the exact credit card that was used at the time of booking)
2/ If you would like the money to be refunded prior to your visit, I would recommend that you call and speak to a Cast Member on (407) 939-3463 and explain the situation. Although this can be a lengthy process, it is possible to alter your existing dining reservation and in this case, your credit card would be refunded straight away.
Wishing you a magical trip filed with memories which will last a lifetime!