The designers of the SP games were Gary Grigsby and Keith Brors, working for SSI. They gave Don and Andy the code (I think), to mod as they saw fit, so long as they didn't make any money of it. So what you really have paid for is a printed manual, and the game burned on a CD. Because Don and Andy have all the power, this is not a democracy like Athens, but more like a monarchy, like Sparta, which had two kings BTW.
After all, the purchased version have nothing essential. Some better resolutions, and some utilities, which mainly helps scenario-designers. I have not seen any difference between the CD-version and the download-version, when playing the game.
Halstein.