DX9 and DX10 are two different installs. One can not overwrite the other.
Unless there is some kind of weird fluke.
From Chuck Walbourn
Technical clarification: Windows Vista's runtime supports Direct3D9,
Direct3D9Ex, Direct3D10, and older interfaces. DXDiag reports it as "version
10" for simplicity, but that number has been meaningless for a few years
now. The Windows Vista runtime also supports all the other usual DirectX
9.0c interfaces except DirectPlay Voice and Direct3D Retained Mode which
were removed. When you run the DirectX End-User Runtime installer or a game
uses the DirectSetup REDIST on Windows Vista (or XP SP 2 for that matter)
nothing in the runtime gets updated or installed. Only the SDK optional
components like D3DX9, D3DX10, XACT, XINPUT, etc. get installed.
The new Aero desktop uses Direct3D9Ex. This API requires Windows Vista, a
DX9 card, and a WDDM driver. It does NOT require Direct3D 10.
The Direct3D 10 API requires Windows Vista and a Direct3D 10 video card. No
released game creates a Direct3D 10 device at this time, although a number
of them have been announced as coming in future products and/or patches.
If you are missing any version of D3DX due to a
failed or badly desigend installer, run the DirectX End-User Runtime
Installer on the Microsoft site and it will install all the supporting SDK
components we've released. Got to
http://www.microsoft.com/directx/ and
follow the link for "Get the Latest DirectX Here".