It's been a couple years since I looked at it but I seem to recall that DX's interfaces changed after version 8, so it would not be easy to go from D3D 6 to D3D 9. And of course there's always D3D 10 which would eliminate those who are still on XP, but that's a different discussion.
The real question is whether or not WZ 1.10 would indeed benefit from a more recent version of DirectX. The answer is a firm and resounding NO because as it is, WZ 1.10 doesn't even utilize all of the features of DX6 (or even a substantial subset of it). Warzone's rendering engine is actually very, extremely basic in its capabilities. Just about the most fancy thing it does is transparency effects on the menus
On top of that, the engine itself is ponderously inefficient.