If you have the same image on both controllers, you should be able to manipulate where they connect by changing the primary controller settings...
config ap primary-base Controller_name AP_name Controller_IP
If you make this change it will overwrite the existing Primary Controller configured on the AP (please be aware), but should allow for the "graceful" migration you requested. (if the priority is already set, which it appears to be from your post, you may need the additional step below)
If the AP does not migrate, you could try changing the "priority" of the AP, which would force a lower priority AP off the controller (and presumably onto the other controller)...
config ap priority 4 AP_name
For this to work, the existing APs would have to be at a lower priority. If they are not, you can use a similar command for the APs you want to move off the primary but using a number lower than 4. I've had success with this method on a number of release versions in the past when needing to juggle APs on 5508 controllers without rebooting them. Hope this works for you.
PS
These commands are one AP at a time so you will have to build a list of APs. The quickest I've found is to make sure your screen is set wide enough for the show ap summary output with one AP per line and issue these commands...
config paging disable
show ap summary
Then grab the session output and use a text editor with block mode editing (like Notepad++) to strip the unnecessary columns and insert the commands in front of the column of AP names. The process to build the commands for 500 APs can literally be done in minutes (<2).
