I have always been overly cautious when I do maintenance. I take a backup of the Gaia config. A backup from within Gaia, and a snapshot too. Just make sure they are all copied off box in the event of a complete failure. I/E: hard drive. It has never happened to me, but I have always been more safe than sorry.
So as for the procedure on upgrading. I generally recommend doing the maintenance on the standby member first. Leverage CPUSE to do the install for you. The process is very straight forward and clean. Use sk92449 to make sure you have the latest build of CPUSE. Really early versions of R77.30 didn't always upgrade on their own. After the JHF has installed, the gateway will reboot. You can manually fail the cluster member over and test inbound/outbound connectivity. If everything tests ok. Repeat the process on the other (now standby) member.