See this thread:
http://www.virtualmin.com/index.php?option=com_fireboard&Itemid=77&func=...
You'll need to skip up a bit, as this link is pretty deep into the conversation.
In short, RPM is failing to install wbm-virtual-server, because of the postinstall script detects an existing installation.
If it's safe to cross-grade this way, as we've been doing for a couple of years in the Professional packages, perhaps allow-overwrite should be enabled for the GPL packages. (Also remember that allow-overwrite was broken until fixed at YAPC in the deb maker...I think the same bug is/was present in the RPM script, but I've always had it hard-coded to remove the post-install script altogether, so I'm not sure.)
If not, we need to document how folks can cross-grade. I'm not sure why wbm installations are still so common...I need to spend some time coming up with a short installation guide for the Webmin site, in addition to the one we have at Virtualmin.com...people are ignoring what's on the Virtualmin site when they install GPL. I guess, assuming that all they need to know is what's on Webmin.com.