Works for me on Windows 8 with 64-bit without any needed changes.
Windows 8 does allow you to emulate earlier versions of Windows, which I use for some games. But, this was not needed for WW1. However, I did have issues with the demo. Once I used the full version it worked grandly. Just make sure you update to the latest patch.
I am totally enjoying play on Windows 8. If it looks interesting, which it should, go for it. If you find some tech issues, which I doubt, a fix will be very easy.