FYI, there is no dx9 compatibility option in dx11. it is a superset of dx10, meaning that dx10 hardware can use the dx10 base of the engine and won't implement the extra dx11 features.
currently you write a dx9 version of a game, and then write a separate version for dx10. there is some overlap, but they are fundamentally different. dx11 is better thought of as dx10+. the card/hardware can execute the 10code and ignores the +code. there is no option to convert it to dx9.
msdn said:Direct3D 11 is designed to support both new and existing hardware, from DirectX 9 to DirectX 11, as shown in the diagram below.
