Do you have the Admin Pack license key installed that allows you to back up the Management Module configuration?
If not, make sure you have an extensive set of notes on everything from storage to networking. Especially if you have set up advanced networking on the switch(es).
If running a Hyper-V Cluster on this setup shut down your HA VMs prior to running the firmware update. That way as the Compute Modules go up and down for the firmware/BIOS updates on them your VMs won't try and move back or get hung up in some way.
And one more thing: Having a remotely managed PDU or UPS system that can allow you to remote power-off all of the power supplies is a good thing to have...just-in-case.