In addition to the two documents cited previously, we also have an October 22, 1970, gun application that features the double-A spelling and the signature of Elvis A. Presley.  This proves conclusively that Elvis was using the name "Aaron" long before Vernon used the double-A spelling on the grave marker.
UPDATE (9/27/10):  Another example of the double-A spelling being used prior to August 1977 is posted below.  This is from a Quitclaim Deed signed by Priscilla Presley on August 15, 1972.