Upgrade/Downgrade Firmware Cluster Best Practice

SW INFO: -Fortigate 6.2.2 vm on Eve-NG Nov 2019

-FW1 and FW2 HA A-A

QUESTIONS

1. This link said must do full backup. Any reason why not do standard backup?

https://help.fortinet.com/fos60hlp/60/Content/FortiOS/fortigate-best-practices/Firmware/Performing_Firmware_Upgrade.htm?Highlight=upgrade%20firmware

2. When cluster upgrade/downgrade firmware. It will upgrade primary and all the slaves.

Most of the time ADMIN need to test impact of newer firmware.

Which option do you think better

A. upgrade/downgrade all cluster member

B. disjoint and make standalone a cluster member

make standalone firewall the same config as cluster

upgrade standalone firmware

shutdown cluster member

test standalone forti

if latest firmware no problem

I am not sure for the next steps what will happen when latest firmware of standalone forti join cluster with old firmware member

should I downgrade firmware of standalone forti first, then join cluster then upgrade?

tq