How to get my FortiAP U423EV out of meru code and connect to my FortiGate on its code

i have 2 new FortiGate's and around 400 fortiap FAPU431F's connecting fine.  i need to get a couple of my old U423EV's to connect to the new FortiGate's.  they were connecting to Meru 4200 controller.  i changed the AC ip but it doesnt upgrade or connect to the new FortiGate's.