No need to uninstall DX9,as.. well, you can't! It's not possible without a reformat

[EDIT: You said "reinstall" not "uninstall" -- no need to. I've swapped out cards before and left DX as-is. It's totally separate anyways, so you should be good!]
The settings in AH are independent of your card. You might want to turn them up, once inside AH, if the card can handle more than your old one.
Be sure to install the proper drivers for the new card, and as long as everything is hunky-dory in Windows, AH should be slicker'n snot.