"Some drivers only support one version of DirectX. But DirectX is backward compatible, which means that newer versions support the older versions. For example, if one has DirectX 9 installed on one's system and runs a game that was written for DirectX 6, it should still work. The game will use what is called the DirectX 6 "interface." Every version of DirectX must support every previous version of DirectX. This is a positive consequence of the COM model used for this API."http://en.wikipedia.org/wiki/DirectX
DirectX 9 cards will be supported under DirectX 10, but only with DirectX 9 functionality. Saying they "won't be compatible at all" is wrong.