@dvhenry
It might be out of topic, but as long as nobody complains 
My solution may not be exactly what you are after when I read “preferably, only some chosen config files”. Starting point is having a separate partion for /home, as advised often here on the forums. Thus it is the whole of /home that is available in both systems.
I have four partition:
. Swap, to be used by both systems;
. SystemA, where the “old” openSUSE is installed (root partition);
. SystemB. where the “new” openSUSE is installed (root partition);
. Home, used by both systems mounted at /home.
Of course this is multi boot (using GRUB). When the “new” system is stable and conversion complete, the boot entry for “old” may be removed from Grub. When in a year or so we go for the “next” openSUSE, it will be installed on SystemA and we will cycle back.
When testing “new” I mount SystemA on /mnt/oldroot readonly. Readonly to protect against any changes. Now when I have a problem in e.g. my DNS configuration, I could see what is the difference between “old” and “new” with:
diff /etc/resolv.conf /mnt/oldroot/etc/resolv.conf
I hope you understand the principles here.
The end-user(s) files are the same on both systems, thus their personal FF, Amarok, KDE, etc. configurations are the same on both systems. Same of course for their documents, music, etc. But there is no choice of: yes for he FF config, but no for the Amarok ones.
There could be some negative effects. My last conversion was from 10.3 to 11.2 and that included KDE 3 to KDE 4. Now in openSUSE the two KDEs have two different sets of personal config file (in ~/.kde and ~/.kde4). I think this avoided a lot of problems when you boot intermittent into both systems. But I remember e.g. that KDE 4 connected a new icon to the Dustbin on the Desktop. Going back to 10.3/KDE3 that icon was not found and a default icon was shown. Nothing dramatic. But it is good to remind that the first time you start a new level of a program, it will find the old config file and it might convert this automaticaly to a new version. Thus when you the you the old program again (because you booted “old”) there could be a problem. Mostly these things are for- and backward compatible though.
I hope I made thhis a bit clear to you. More questions? Please feel free to ask.