How to install Xfce and remove default DE?
Installing XFCE should be easy enough.
Removing another DE is hard, and will get you into dependency hell. Not worth the effort, in my opinion.
If you want only XFCE, you should be able to do that with an install from the DVD. At an early screen it asks you to select a desktop. KDE is preselected. But you can instead choose “Gnome” or “Other”. You should choose “Other” at that point.
It has been around 18 months since I last did that. Using that method, I successfully installed XFCE as the desktop in 11.3 at that time. I don’t recall the other steps after checking “Other”, but I seem to recall that they were obvious enough.
If you already have an installed system, and are not too tight on disk space, why not just add XFCE and leave the other desktop there. If you already have KDE and add XFCE, I think it takes less than 1G. If you already have Gnome and add XFCE, that should take less disk space since XFCE uses a number of the gnome applications. To just add XFCE, get into Yast software management, and select “Patterns”. From there, select the pattern for XFCE.