Upgrade Rule 1: BACKUP!
I'd like to take a moment and remind everyone of the first rule of software upgrades ... no mater how confident you are of your upgrade procedure, it is absolutely critical that you ...
BACKUP YOUR DATA!
I'm currently dealing with a situation where someone did not do a backup before performing a upgrade, and the upgrade had some kind of glitch, and they have been down for 2 days now.
If they had done a backup, like the instructions indicated, we could have restored the backup and retried the upgrade, saving a lot of work a number of fronts.
So, please, do a backup before you run your upgrade. It will save you a lot of headaches in the long run. It doesn't mater what platform you're using ... iSeries, Windows, *nix, Apple, etc. Do a backup.
On the iSeries I recommend you backup the appropriate libraries to a save file with compression and save access paths (so the restore will go faster).