Most distros come with switchdesk, if your distro does not then find the information (read the release notes or visit the web site for your distro for this info).
Basically switchdesk is used to switch the default desktop as in; switchdesk KDE
Sometimes you may have to manually edit the ~.xclients and/or ~.xclients-default file and state which desktop to use, example;
WMPATH="/usr/bin /opt/bin /usr/local/bin /usr/X11R6/bin"
for p in $WMPATH ; do
[ -x $p/$WM ] && exec $p/$WM