Upon purchasing a license for VM Pro, I ran as root:
wget -O install.sh http://software.virtualmin.com/cgi-bin/install.cgi?serial=...
to upgrade from the GPL version.
It reported no errors, but on logging in on port 1000, it took me to a webmin page without the virtualmin options.
The only mentions I see for virtualmin are:
Servers -> Virtualmin Mailman Mailing Lists Virtualmin Subversion Repositories Virtualmin Virtual Servers (GPL)
System -> Virtualmin Package Updates
No change that I saw upon rebooting the system.