You don't actually book a new reservation, but availability has to exist such that you could book a new discounted room. There is no advantage to having a room booked previously when trying to access a discount at a particular resort. Discounts on rooms are essentially a sale on unsold rooms, so, if anything, every room already booked is a slight reduction in the chance of a discount.
That said, if you want a particular room and category regardless of whether a discount is offered or not, then you should go ahead and book it. Rooms and categories sell out and discounts are never a guarantee.
Our strategy is to immediately book what we are happy with if there are no discounts, then we change the booking if a discount is offered later for something that we would prefer/is now in our budget. You don't have to cancel and rebook even when changing, you just pay the difference in the night's deposit (if any).
Good luck!