You may run into problems trying to downgrade the firmware as the master is already on that firmware.
I would say you have two choices:
1. Shut-down HA2, upgrade HA1 then power-on HA2.
This would of course mean an outage when HA1 reboots to complete the firmware update.
2. Fail-over so HA2 is primary. Disconnect cables to HA1 and upgrade, then re-connect cables.
There should be no outage with this method.