There's really nothing to screw up. Even if it is the same driver, you should reinstall it after replacing the card. There are a number of operations that occur during the installation of a driver (including testing the new hw, making sure the system sees and recognizes the new card, and enabling any new features the card may have). Whether or not it's absolutely necessary, it's good practice - and there's certainly no downside to doing so.
Uninstall the old driver (http://support.amd.com/us/kbarticles/Pages/GPU57RemoveOldGraphicsDrivers.aspx
Then either install the driver on the cd that comes with the new card or go to the ATI website, download the latest driver for your card, and install it.
If you are concerned about ANYTHING any time you are getting ready to install new hardware or software, you should create a backup image of your drive. Then you can do pretty much anything you like and you'll always have a safe haven. EVERYONE should use disc imaging software on a regular basis anyay.