You are welcome to take some insignificant risk and install it using development version of install.sh script, which should work just fine. Before installing, and after downloading linked install.sh script, replace SERIAL and KEY with those purchased ones, which can be found in virtualmin.com → My Account → Software Licenses page.
If it was already installed, you could use mentioned pre-release (devel) version of the virtualmin-install.sh script and run it with --setup param to setup repos and exit. You may need to edit the script manually and put your actual SERIAL and KEY in it. Afterwards running apt-get upgrade should install Virtualmin Pro on your system.
Currently there is a bug in Virtualmin GPL that will prevent upgrades from GPL to Pro version using UI. It will be fixed in the next Virtualmin release.