So I've fine-tuned my Natty with different icon sets, themes, fonts from sites I don't remember now. And there are these apps, ppas which are very difficult to find, add again. So how do I backup everything and restore ? How different will an upgrade be from a clean install?
Well if you have a different partition for /home things are a lot easier.
1-)fonts, themes, icons, firefox bookmarks are all in your home folder 2-)ppas, apps and drivers are in / partition so you need to save them in some other way
Start the installation and choose advanced partition or something like thta. In your existing /home set up a mount point for /home and DON'T select to format it, then choose another partition for / and format it.
When the installation finish you can restart and you will have your theme, fonts, settings, account and documents like before.
For apps you can use OneConf which is avalaible in the software center and preinstalled in oneiric
For driver and ppas i don't know
Use partimage or fsarchiver to archive the whole partition. In case you don't like oneiric you can restore the whole partition very quickly. These programs are part of SystemRescueCd.