Have your thoughts solidified? It seems like a simple script would help:
- Create a settings backup directory
- Move the minimal set of settings files into it (based on what is needed to get the newer behavior but not loose serious user customization
- Echo some advice for what the user needs to do post upgrade
The hard part is the second item. There is a lot of stuff in the .gconf and .gnome directories. (I think you mean .gnome2 btw, right?)
Thinking about this, though, I think it might be easier if instead there were a post-update script that would selectively modify just
the settings needed to get the newer look-and-feel. There just seems to be so much that could go wrong with the other way around.
Or, keeping things simple, if there were just a short list of "things to do" to get the new look and feel, we could do it manually. Things like art and menus for example...