They only allot so many rooms for the AAA discount, so once they are booked, there will be no more available, unless someone cancels. If you're going to use AAA discount, it's better to book your room as soon as you can so you don't miss out. I'm currently waiting for the 2007 AAA Discounts (room-only) to come out so I can grab it
To directly answer your question, if you book the concierge room and a AAA discount for a concierge room becomes available again (if someone cancels) then your travel agent can ask to have it applied. The thing is, once the AAA discounts are all booked up it's been my experience that they rarely become available again, however it's not impossible.