This article describes how to change FortiAP-U image from FortiGate to FortiWLC.
Step 1: Get CLI access to the FortiAP which is booting with FortiGate image.
Step 2: Execute the below command:
PU431F5xxxxxx# factoryreset
This operation will reset the system to factory default!!
Do you want to continue? (y/n)
Step 3: The FortiAP will go for reboot and once it comes up, it will come up with a FortiWLC image.
Now the FortiAP will be able to discover FortiWLC and come up online.