If I understood you correctly you wish to change which desktop environment you login to by default, I presume you now run the XDM instead of KDM/GDM and hence you might not swap between the DEs as easily.
You may change the default DE you log in to here: YaST Control Centre > System > /etc/sysconfig Editor > Desktop > Window Manager > Default WM (if openbox is not available as an preset option just type in ‘openbox’)