Lots of questions…say I have a clean, new CentOS install on the new server- how to proceed?
It’s actually pretty straight forward. Assuming you have Virtualmin installed on both systems, generate a set of backups of all your Virtual Servers on your existing server, copy them to the new server, then perform an import. After that, all your Virtual Servers and your Virtualmin config info should be migrated to the new server.
Then, it’s just a matter of testing it out before you make those new sites live.
Is there an automated or single tweak way to flush all the Junk Mail folders before the move? Any other stuff I can purge?
Well, not that I know of… though, since the backups compress data, I’m not sure that the amount of space junk mail takes up would make it worth your while to get rid of it.
Can I run 2 copies of the same Vmin license for a short time?
Sure! The Virtualmin license allows for you to run two copies for things like migrations.
Then, “just” use Backup Virtual Servers to send them across one at a time, and restore on the other end?
You can do them one at a time, or all at once, depending on your goals. To do them all at once, put all your backup archives into a single directory, and tell Virtualmin to import that entire directory.