If a discount or special is offered you will have to call to have it applied to your current reservation, but already having a reservation does not guarantee that the special can be applied. Your resort, room category and dates would have to be included in the special and there would have to be availability. Sometimes only a limited number of rooms are offered and they go quickly so you might have to change room category or resort to take advantage of the special.
There is a Disney Resorts forum and a sub-board for Disney Discount Codes and Rates that you can keep an eye on.