This is ifnormation about d x 11 to 9 posted on the net --->
This backwards compatibility doesnot mean that all your games will run out of the box, they depend from a "fitting" DX environment, so the installation of a DirectX version that came with the game or - preferrably, as it saves time - of the DX WebInstaller is necessary.
As dx 11 contains the core of dx 9 but not all that dx 9 got.