Hello, I'm a total newbie about game programming and DirectX, I know C++ and I'd like to learn DirectX, at the same time I'd like to contribute to EECH development community improving this Helicopter Flight Sim (its source code was released). The game was made using Direct3D (DX 7.0), my question is should I learn Direct3D in DX 7 or Direct3D in DX 9 ? Thanks

If you want to work on a DX7 codebase, then you need to learn DX7. You can always learn the new features and other changes in DX9 later.