That's what the CM told us last week when we called and made reservations. That we could call and they would just apply the discount. But I don't know how true that is as I've also heard that you have to completely cancel your reservation and get a new one.
I've called many times to change rates....rack to AAA, AAA to AP, rack to AP....never a problem.
I always have gotten an updated document and it has always been the same confirmation number.