What happens if my subscription for Pro expires?

You'd want to downgrade to the GPL version. You won't lose your sites, at all.

Exactly how you downgrade depends on your OS, and how you installed. Usually it's just a matter of installing the GPL RPM or deb or .wbm file (depending on how Professional was installed) and converting the package manager to the GPL version of the repository--this may not be perfectly possible, if you're using an OS not supported by GPL that is supported by Professional, but the recent versions of the most popular three operating systems (CentOS, Debian, and Ubuntu LTS) are supported in Virtualmin GPL.

If your installation is RPM-based, you'll have to use the "--old-package" option to RPM to install the GPL version. Because upgrading is the far more common path, Professional has a higher Epoch than GPL, which means it always looks like a newer version to rpm/yum, even if it isn't.

The GPL repository has the exact same URL as the Professional one, except it has "/gpl/" at the beginning of the path.

So, on CentOS;

http://software.virtualmin.com/gpl/centos/5/i386

Obviously, with GPL, you don't need a serial number or license key in the URL.