Upgrading Active-Standby LTM into v11.5.3

You can upgrade the standby unit first, check if the configuration worked completely with the new version (you may have problem from 10 to 11) and, if everything were OK with the just updated device, you can switch it to active mode and then update the other one. The synchronization does not work between different versions, but the fail-over do work! Once both devices are upgraded, you can manually sync the configuration and get everything back to green. Only avoid using auto-sync during these operations.