changing from package to room only or vice versa isnt simply keying in a different code. They cancel the one, refund your deposit, and book a new reservation and take a new deposit. Sometimes that runs rather smoothly without the guest even realizing what is really happening. Sometimes there can be some snags.
If you want to keep your package, contact your (or any since some are much better at booking Disney than others) aaa office. You can get a package with a aaa discount (still just the exact same amount you would have saved with just a room) but only through your aaa office. It is possible for you to sign your reservation over to aaa to aquire the discount. I don't quite know how it works but aaa would. Or if you are more than 45 days out, they can just book you a new vacation and you can cancel the original one.
AAA rooms are limited so it is not a guarantee that you can get a savings. And talk has been that there are x number of aaa rooms and y number of aaa packages. It is possible that you might would be able to get a aaa room only when there are no aaa packages available, or that you might be able to get a aaa package when there are no aaa room only's available.