I just can't see what's particular risky in this and it's a bit tough and time consuming to backup ~500Gb and then restore them back. That's why i hope there's nothing too dangerous in a small os update .
Hi, there is ALWAYS something that you didn't thought of that could go wrong. That's Murphy's law.
Just put it this way : what if you loose all of your GF's data ? I bet you'd feel like a fool not having spent those few hours protecting that precious loved one's data.
It's better to be safe than sorry. Take it from the guys here that gave you this advice. ALWAYS BACKUP FIRST !