Upgrading controller will not upgrade your APs until you go to each zone and change the AP zone firmware.
For example, if you upgrade controller to 5.2.0, AP zone will still use the 3.6.2 version and you have to go to Zone >> More >> Change AP Firmware.
This gives you freedom to upgrade AP firmware as per your convenience and also you can run different AP zones with different AP firmware.
Each zone acts as dedicated controller (assume each Zone as a Zone Director, if you are familiar with our ZD products).
Syamantak Omer | Community moderator | Staff TSE | CWNA | CCNA | RASZA | RICXI | Follow me at https://linkedin.com/in/syamantak-omer-5949b6124