The API is supposed to be backwards compatible with standing architecture, either by emulation or by direct operation. Meaning that if you have DX10 installed, anything written in DX9 to DX3 should work 'out of the box'. That was the point behind having the DX unified architecture in the first place.
If it's not doing that, there's something wrong. Very wrong, either in the drivers(video/audio/whatever) or the API itself. It really sounds like someone screwed the pooch on the API releases.
Correct--which is why my DX9 graphics card ran all of my DX9 games without a problem when I first upgraded to Vista running DX10.
But all of this talk about "DX9" mystifies me. I updated with this file this morning and saw nothing anywhere that mentions DX9. Rather, what this patch does is to bring Vista up to the level of Win7 in terms of the DX API--DX11. It also brings 2d hardware acceleration to Vista--which under DX10 Vista did not have. That's it as far as I can see.
Just for fun the other day I installed Legend's Wheel of Time, a game that shipped in 1999, into my DX10.1 Vista installation running on my 4850 Crossfires--and without any software modification the game ran wonderfully well for me--better I think than it originally ran when I bought it. Which proves your point pretty well...;) There's a lot of confusion on this issue.
It is well known that I cannot err--and so, if you should happen across an error in anything I have written you can be absolutely sure that *I* did not write it!...;)