If I've already booked two ADR's and THEN I decide to add on the DDP, do I have to call back to the dining line to mention this or will it work itself out when I get there?
The only reason you might want to call back is if you have a reservation for something you had to pay in full at booking for (Cinderella's Royal Table or the dinner shows - Hoop De Doo, Spirit of Aloha or Mickey's Backyard Barbecue) to make sure they credit you for the payment now that you will be using the dining plan.
Otherwise, it makes absolutely no difference whether or not you are on the plan when you make reservations.
They ask when you make them because they want to explain the plan to you, in case you are booking a restaurant that costs 2 credits or something like that.
The restaurant will ask again when you get there, so they will know how your order should be rung up.
If I've already booked two ADR's and THEN I decide to add on the DDP, do I have to call back to the dining line to mention this or will it work itself out when I get there?