I do not want to sound like a difficult person, but Moodle is not built to work with Windows. We struggled for at least four, and maybe five, years to get it *out* of Windows. It really is a bad idea to leave Moodle's native environment and go to Windows. I can't even list all the trouble we had.
That aside, you just need to make a server for the previous versions and maintain it for the appropriate length of time.
Easier to stay with a Linux server than to go through the pain I'm imagining you are going through. Or, anticipating going through.
If you've got the server, why would you want to leave it?
Hence leave it or crash and burn irrevocably at some point and as I look after Moodle I can't live with that kind of strain, thank you - spread the load.
Can't leave the server up and running because it becomes too costly in licence fees for the back up systems etc.
Nothing is ever easy. Moodle might be open source but on a Windows college everything else is licence fees.