THe latest code has a new feature described below. Does that help with the issue of moving all APs off a controller prior to upgrading the controller. If so it would be very welcome.
Also, the lastest MD code is 18.104.22.168. I have been running 22.214.171.124 due to our 1142 APs. It seems like it is taking a long to get any 5.X or 6.X MD code.
AP Image Predownload
This feature allows you to download the upgrade image to the controller, then download the image to the access points while the network is still up.