There seems to be a misconception that if you have a room reservation and a discount code comes out you can call and "switch" to that code.
When a rooms are discounted, only a limited amount of rooms are available with the discount. Once these rooms are booked they are gone. So the fact that you have a room at rack rate does not guarantee that you will get a room at the discounted rate.
If you are committed to going on certain dates I recommend that you reserve your room and hope rates come out and that you can get them.
We always budget to pay full rate and use any discounts to upgrade our room.
One other thing. For those people who are in the habit of booking rooms and later canceling or changing dates. When a discounted room is booked it comes out of availability for discounted rooms. If it is canceled it goes back into general (non-discounted) availability.