After upgrading our Virtual SmartZone and all AP Zones to 5.2, we ran into an issue with the Captive Portal on our Guest SSID's no longer working on R730's and R610's. I don't know if any other model had the same issue, but after downgrading the AP firmware back to 5.1.2, Captive Portal was working again. We had tried factory resetting the AP's, moving them to a different controller on same firmware, deleting and re-creating the SSID's, etc.
Support is working to replicate the issue, but wanted to make sure people knew what we ran into in case you see it as well.
I typically go against the norm and run pretty bleeding edge. We do have 2 separate controllers, so I will usually upgrade one within a week of new firmware being released, let it sit and then upgrade the other one within a week after that if no issues are found. I upgrade the APs as soon as possible especially since we are running some of the new R730's and every new release has quite a few more features for the new AX AP's.
I don't know about scheduling a time for the upgrade, but one of the BEST improvements to 5.2 is you can now upgrade every AP Zone at the same time rather than going one Zone at a time! That will save a LOT of time in the future. Luckily we were able to keep the controller on 5.2 and just downgrade the AP firmware back to 5.1.2 for the time being.