Yes, if there is availability, you can add a discount to any existing reservation, even if paid in full. From other people's experiences, they don't apply the discount back to you though, they give you a credit on your room--for charges. If you still have a credit when you check out, THEN they will issue you you a credit--and it *can* take up to 60 days before you receive it. It doesn't usually take that long, but they warn you it can.