if you are using astaro security gateway software and not an astaro appliance it is as simple as you described. download the latest iso-file, burn on cd, install v6 and restore backup.
In theory the upgrade is very simple and straight forward. The only precaution I would recommend would be noting down the MAC addresses of your NICs prior to doing anything. I have, in the past, encountered the situation where when doing an update/reinstall the NICs were assigned differently. This is not a real issue, if you know it has happened, but if you don’t it can cause some real angst.
