I have been playing with GPL and quite like it. However there are a couple of things in Pro that I'd like to play with too (mainly script installers for things I really can't be bothered to manage myself!).
As I understand it, if I upgrade my server to Pro I can't go back? Therefore I plan on setting up a virtual server to test with. Is there any way to "trial" the pro version or do I just need to buy a license? If so, is it "acceptable" to buy the licence with every intention of claiming the money back under the 45-day money back guarantee? (Even if I decide to keep Pro, I won't want it on my virtual server!)
Also: I can find anything to tell me whether domain aliases count in the domain count for licensing? Assuming they don't, Pro is quite cheap, but if having multiple URLs pointing to the same site count towards the total it would get expensive pretty quickly!
OK, I think I've answered some of this myself...
Downgrading is possible: https://www.virtualmin.com/documentation/installation/uninstalling
Therefore as my server is only a test server I think I'm OK to upgrade to Pro then downgrade if I don't like it.
The only original question remaining is: how is the license calculated (do sub-servers count)?
And another question: What happens if I let the Pro licence lapse: will things "break" or will I get plenty of time to renew the licence or downgrade?
You can easily transfer the license to another physical server when you have finished your experiments, no need to re-claim the money and buy a new license for another server. :)
The license manager will mention 2 active installations for a while then, until the one on the no-longer-used virtual server times out. Also, the license explicitly entitles you to a second installation on an experimental or backup server, as long as it does not serve stuff for the public.
Alias servers don't count towards the domain limit, since they are no real "full servers" but just another name for an existing server. Sub-servers do count though, since they are "full servers", they just share the administrative account with their parent. (Note that in your first post you mentioned "alias" and in your second you mentioned "sub".)
When your license lapses, your server should continue operating just fine. You (and the developers) will just get a license warning -- at least as far as I understand previous statements by the developers that they never deactivate installations.