Well, first of all the Steam release doesn't just have the original game; there's some bonus content. Specifically, a set of avatars, the game's soundtrack in MP3 format, and a Tex Murphy trivia game. Then of course there's the additional coding required to get the game to run on modern operating systems. So even if you could prove you have the disks, I doubt Nightdive would feel unjustified in charging someone who owns the original game for the Steam release.

To install Under a Killing Moon on Windows 10, you'd need a DOS emulator. I think DOSBox is still the premier DOS emulator, but I'm not big into DOS emulation so don't take my word for it. At any rate, DOSBox's entry for Under a Killing Moon[www.dosbox.com] lists the game as fully supported. That page also has some forum posts which might help you with the installation and mounting.

Just install it in DOSBox. The Steam version uses it anyway, just with the files reorganised to run off the hard drive. If you rip the discs to ISOs, you could mount them as four drives in DOSBox and not have to do any disc swapping.

I already had UAKM and The Pandora Directive and just bought the franchise pack to get the rest of the series in one go. Don't expect to have an easy time getting Overseer working, though. It's notorious for not liking anything made after it was. Even the nigh-magical Proton, which gets many finicky old Windows games working flawlessly to the point that it's easier to get them working by just running them on Linux through Proton than to play on modern versions of Windows, doesn't manage to get it working properly.