I may have missed what you were asking. In your /etc/X11/xorg.conf and the xorg.pluto.conf you can manually change the drivers from ati to fglrx and reboot or restart X. If that doesn't work, you can always change them back to ATI. You might also wish to edit the cards file (you'll have to search for forums for the full path, my memory hasn't had a full cup of coffee yet) and change any reference of the ati to fglrx or visa-versa.
Maybe taking a step backwards, but what issues are you seeing that are making you want to change the driver, exactly and where do you want the driver changed. 9200 vs 9250, is there a significant difference in the cards that you're not getting the results you wish with the 9200 driver?