HI,
It is possible through the Controller.
Step 1 : bring up the RAP and terminate it on the controller which is running with the desired image version (6.3 or 6.4 in your case)
Step 2 : on the controller, use the following commands to upgrade the backup partition of the RAP
(Aruba3600) #apflash ap-nameTEST-AP backup-partition
You can verify the same by using
(Aruba3600) ##show ap image version
Hope you got your answer, please feel free for any further query on this.