The process I’d use would go something like this
Install Opensuse
Run updates
Boot off a live disk
Copy the contents of your installed opensuse partition to some appropriate location
Boot back into the installed opensuse
Upgrade your gnome to version 3
Note: When copying the contents of your installed opensuse partition there are some folders that can and perhaps even should be omitted (meaning don’t copy them), here is a list of those I don’t copy, other people may have other suggestions on this
/dev
/home - Contains user profiles & data, /home I back up on a regular basis anyway using remote rsync differential backups (a whole other subject entirely)
/media
/mnt
/proc
/root
/sys
/srv - Contains webpages etc if you run a webserver, if the machine runs a webserver I would be backing this folder up regularly also
I would do the copying in one of two ways, either a command something like this for each folder to be backed up:
sudo cp -a -u -v /bin /backup-location
Or, a tar command similar to this:
sudo tar -czvspf /backup-location.tar.gz /bin /boot/ /etc /lib /lib64 /opt /sbin /tmp /usr /var (the folders you need to tar may differ slightly)
Or, if you prerred you could use Yast > System > System Backup
If the gnome 3 install fails and you aren’t able to revert back to your previous gnome for some reason you would then have the backup which you could write back using the live disk and (hopefully) revert the system back to how it was before you installed gnome 3
Bear in mind I’ve never installed gnome 3 but if I were going to I would do it along the lines of how I described above
Incidentally I didn’t know there was a live opensuse disk with gnome 3, and I don’t know how gnome looks on opensuse these days, but the image you posted looks more like fedora to me than opensuse