If the system is running Vista or later, don't bother using a driver sweeping program, just uninstall, restart and install the AMD drivers. If it's on XP or earlier then yes, it's a good idea to use a driver sweeper.
And, in order:
Download the latest WHQL certified drivers for your operating system and card but save them (don't use the driver disc, it's a LONG way out of date by now).
Uninstall the existing drivers.
Power down (and remove the power cord-yes, this is important).
Swap cards.
Power up.
DON'T PANIC! The display is supposed to look like that! With no hardware drivers Windows has just defaulted to its own built in, low rez VGA drivers.
Install the drivers you downloaded.
Restart.
It may be that you'll need to adjust the screen settings through the Windows Control Panel after the first start.