Sometime it's hard to say if you will run into issue during upgrade or not as each ISE deployment is different. Having upgrades multiple times, I normally follow Cisco procedure until the last node, which is the PAM, where I would just wipe the server and join a fresh node to the upgraded deployment, as I once ran into problem where the final node cause the entire upgrade to fail. Technically, you only need to upgrade the first node (ie. Secondary AM), do fresh install on every other nodes, and register them from scratch. I have also heard from Cisco TAC this is a recommended way. If you can, try to upgrade in the lab and don't forget to take a good config backup or VM snapshot.