You book your room/package using the regular rate, and then once a code comes through, or you get a AAA, AP, or Disney Visa rate you either change your rate by calling, or you cancel your reservation and make a new one with the code/AAA/AP/Disney Visa rate?
Yes that the way you do it. But, the new rate has to be available. They do sell out. Especially AAA rates. You don't need to cancel ressie you have they (CRO or TA) can apply the new discount code to your current ressie.
Correct there are a certain # of rooms available for use with a certain code. No one really knows how many so it is usually advisable to get on the DIS early every AM to check it out.