Command to save the configuration

Ideally, you probably always have the latest configuration in memory saved to disk, because whenever you make LTM changes in the web interface it automatically applies them to both memory and disk. But just in case, if you want to be sure what's in memory is on the disk, save save /sys config partitions all should do it, as you said. But, F5 does not automatically save configuration in memory to disk before rebooting so you would have to run the command before beginning your upgrade.

Just as a precaution though, I would recommend you also run the command above to create a UCS on all boxes being upgraded, and then download them to a safe place. I've been working with F5s for a number of years and have done quite a few upgrades, and on occasion bad things happen and you may want to restore from a UCS backup. It's rare, but better safe than sorry!