Applying a discount is based on availability. And applying it is considered a modification of an existing reservation, so you don't need to "start all over again."
However, there must still be availability under the discount for the room type and resort you have before they could apply the discount without other changes. You may have to change room types (standard to water view, for example) or resorts to get the discount. These changes would still be considered a modification of your reservation.
The only exception is if you're changing a room-only reservation to a package or vice versa. Then it's a new reservation and you have to start all over again. (You can apply a room-only discount to a package reservation; it simply discounts the room portion of the package while you still pay full price for the dining plan and tickets.)
