Hi Aydin
Correct me if I am wrong.I see this as overhead though.
How about upgrading both C4210 and C5210 to code 9.21.16 [common code that is supported by both controller and AP36XX but NOT by 3935]
Then having them in HA pair should help sync configuration from C4210 to C5210.
Move the AP36XX to C5210. once the setup good.Remove C5210 from HA pair,upgrade only the C5210 to latest code to support 3935.
AP39XX does not support code 9.X
AP36XX does not support code 10.x
I have not tested the above,please consider the license factor and Physical port/VLAN configuration.
Regards
Rajesh J