There is information in that maintenance guide that tells you how to reset the license.. Are these procedures not an option?
To restore the system to a particular image with factory defaults, restore the system from rescue mode with the upgrade image as the restore image. For example, if the system needs to be restored to factory default image V9.21, enter the rescue mode, follow the procedure to restore the backup image (as explained in Restoring the Backup Image from Backing Up and Restoring the Image Wireless Maintenance guide 28 Rescue Mode on page 24) and, instead of selecting the backup image, provide the upgrade image V9.21 (local, flash or download from FTP), and perform the restore. The system is restored to the V9.21 with factory default values. Be aware that restoring a system to factory defaults means that the configuration is lost including the IP connectivity, certificates, and licenses. Restoring to factory default is possible only from rescue mode.
Another option, login to GUI from the Admin port (PC 192.168.10.2 if Admin has the default IP)
In the Reset Configuration section, select the appropriate configuration reset options:
• Remove installed license – The system reboots and restores all aspects of the system configuration to the initial settings and the license key is removed. However, the Management IP address is preserved. This permits administrators to remain connected through the Management interface.
The https://extranet.extremenetworks.com/downloads/Pages/WirelessControllers.aspx should explain what all the buttons do on the controllers...
Example:
https://extranet.extremenetworks.com/downloads/Pages/dms.ashx?download=a303946a-ed16-4b2e-8539-a1bdf...
Doug Hyde
Director, Technical Support / Extreme Networks