Ubuntu 12.04 is almost five years old, and that means Canonical will discontinue support for it in a few short weeks. And, that means we will discontinue support for it in Virtualmin (it would be crazy to run a distro on a world-facing server without regular updates). In short: If you still have any Ubuntu 12.04 LTS systems in production, now is the time to upgrade!
It's a few weeks away, which gives you ample time to upgrade to 14.04 LTS or 16.04 LTS (we always recommend the latest supported version of your preferred distribution). Upgrading Ubuntu can be done (mostly) using apt-get, so you can upgrade a server "in-place" without reinstalling the OS. You should, of course, plan for down time, and testing.
Eric wrote up some docs about upgrading from 12.04 to 14.04 here: https://www.virtualmin.com/documentation/system/os/ubuntu-precise-to-trusty
And, some docs for upgrading from 14.04 to 16.04 here (which is a more complicated upgrade than the former): https://www.virtualmin.com/documentation/system/os/ubuntu-trusty-to-xenial
If you really can't upgrade your 12.04 systems, Canonical offers the Extended Security Maintenance to paying customers, which will cover the most dangerous elements of the system.
It is also possible to backup your Virtualmin domains and configuration, and migrate them to a new system with a newer version of the OS (this still needs testing, as your apps may expect older versions of PHP or other packages, and may need some tweaks...Virtualmin handles a lot of that when restoring, but not all of the possibilities, by any means).
Fresh installs of Virtualmin on Ubuntu 12.04 LTS will no longer be available in our install script after the Virtualmin 6 release (coming really soon). And, we will disable our 12.04 LTS repository completely a few months after the OS reaches EOL.