I just wrote this same question in the Slackware forum, thinking that the answer might be the same (I have a slackware system that needs this same problem solved), but apparently it isn't. I was told to use xwmconfig or to edit the ~/.xinitrc file to change my desktop environment, but neither of those works for Red Hat 7.1.
There is no .xinitrc file in my home directory, and I totally cannot understand the /etc/X11/xinit/xinitrc script to make it do what I want either. Any Red Hat specific help on this? Thanks!