Changing the Windoes Manager after install (not through UI)

I installed OpenSUSE 11.1 today and all is well, however I selected KDE4 as the default desktop, I now need to change this to KDE3 or gnome (would like a choice) as KDE4 does not support multiple monitors.

This is easy if you can use init5, but since I can not (it crashes the Video subsystem (related to ATI+NVIDIA on the same system), I want to change the default Windows Manager for the entire system (all present and new users from now on), however there does not appear to be any simple way to do this?

Especially for users that have already logged into a desktop in this case KDE4.