I might be wrong, but I didn't think it was as simple as just "applying the code". The impression I got was that there were a certain number of room types etc that qulaified for the code, so if you had - let's say - a standard view at WL booked and the special was released, it would still have to be available as part of the special rather than just "adding" the code to the existing package. So if there was only Courtyard view available under the special, you'd have to book that.
I hope I'm wrong though! Does anyone know for sure?