A backup of a running system is a good idea. Keep in mind how often we have trouble with new Raspbian versions. It still seems to run best on Jessi, so at least keep an image of that Raspbian version at hand that works for you (but then again, why not a complete backup image?).
My Mirror crashed with data loss and corrupted sd card and for weeks, evening after evening I couldn’t get WiFi to run on my RasPi with the new card and latest Raspbian. Finally found my old Jessi image and it worked immediately. If only I had made a backup, it would have been a 10 minute thing.