Windows considers DirectX9 and DirectX11 to be two different applications. You won't be installing over anything. A lot of programs (like games) use DirectX9 and can't use DirectX11. I think DirectX versions used to be "backwards compatible", but it isn't like that with DirectX11.
You can verify this by doing a google search of "installing directx 9 over directx 11" (although, if you look at forum threads you may have to read through quite a lot to see accurate information).
8 Years Ago by