In a controller managed environment, AP will download the image from the controller to match the controller ArubaOS. As far as that APs in your network is supported on ArubaOS 6.x, it will automatically downgrade itself. If AP is not compatible with ArubaOS 6.x, it will not work.
You can find the minimum software release supported for each AP in its datasheet. E.G. Datasheet for 210 series AP states min ArubaOS release supported is: ArubaOS 6.4.2.0
https://www.arubanetworks.com/assets/ds/DS_AP210Series.pdf