DX sort of encapsulates a lot of different technologies.
No, DX9 should not have any impact on your DX plugins. It might have an impact if you use Direct Sound drivers for your soundcard, but that's rare.
Now I say "should" and "might" because this is Microsoft we're talking about. I game a lot on my machine, which is also a DAW, and I keep up to date on DX. Haven't installed 9 yet though, but I will.
Slackmaster 2000