You can either cancel your ADR and get a refund and then attempt to rebook the ADR after you have added the dining plan (you' will likely have a difficult time getting an ADR again) or you can keep your ADR, add the dining plan and then let the restaurant know when you arrive that you will be using dining plan credits instead and they will refund you then.
Either way if you're getting the dining plan anyways you could have saved a lot of hassle by just doing it before you made your initial ADR's.
Dining plan has to be added at least 3 days before check in. If you add dining plan when making your initial booking, you must pay for it at the 30 day mark.
So, by waiting to add
DDP, you can hold off paying for it for an extra 25 days or so. Could make a difference to some.
The only restaurants that charge you up front, at the time the reservation is made, are:
Cinderella's Royal Table
Hoop de doo
Luau
Mickey's backyard BBQ
Every other restaurant requires the same credit card guarantee, whether on the DDP or not. If you are a no show, it's $10/person. Showing up with less people than the ADR is made for will not result in a no-show charge. You will be asked whether you are using the DDP or not upon arrival at the restaurant (either at the podium or by your server).
For those that charge up front:
If you have the dining plan purchased, it will ask you when making your reservation if you plan on using the dining plan. If you check "yes," you will not have to pay upfront.
If you check "no," or purchase the dining plan after making the ADR, and decide to use your credits for the meal, you will tell them when you check in at the podium that you would like to use the DDP, and they will refund your credit card then.
Alternatively, you can try and book a NEW reservation for the same time. If you are able to make one (and select dining plan as form of payment), you can then cancel the initial reservation. If the old and new reservations are within an hour of each other, it will auto-cancel the old one for you. Your credit card will then be refunded. If you can't find a new reservation, DO NOT cancel the old one hoping to grab it again. Someone very well can beat you to it!
Booking a new rerservation works great for CRT. However, there is a hidden "price" for the dinner shows. Seating is assigned in the order the reservations are made, from front of house to back, but in a way that does not leave empty seats). So, if you book them at 180 days, you will get the best seats. But, booking a new reservation a few days before arrival, and canceling the old, could result in you being given a seat towards the back.