When it doubt about a system level change, make a drive image first. I've done this many times, and every once in awhile it's a major lifesaver (we applied the patch, and it won't boot anymore). This gets more true the more archaic your equipment and OS gets, such as those finicky systems in the corner left over from over a decade ago that everyone's afraid to touch... but that have to be touched for a (security) upgrade or whatnot.

Acronis True Image, Clonezilla, Norton Ghost, pick your poison; but until you know it works, try a restore to a near-identical drive/RAID.

If you're really paranoid, drive image it, pull out the drives, put them on a shelf, put in new drives, restore the image, and _then_ do your upgrade; you know your image works (you just restored it), and you know you have the original drives sitting on a shelf, ready to be popped back in, as well.

Regardless, restore your backups regularly; if you support PITR, _test it_. Maybe someone slipped in a job that truncates the transaction log and nobody's noticed yet.