You certainly don't have to wipe out the operating system just to change any hardware. If you are using a graphic login screen then you only have to do the following:
1- change that to a text console login screen
2- install your new card
3- start the operating system in text mode
4- change your /etc/X11/XF86Config-4 file to match your new card
5- test your settings
6- set your computer back to using the graphic login
Here is how to do that.
1- You could use a boot parameter to boot into run level 1 in which case you could skip this step. I will show you how to do this with the system configuration file named inittab because I'm not overly familiar with passing boot parameters to GRUB, which is what Debian Sarge uses as its default boot loader.
Set your default run level to 1. You do this by editing the file /etc/inittab. First log in as root. Set your current directory to /etc. Copy the original inittab file to inittab.original. Edit the inittab file. Find the line that looks like this.
# The default runlevel.
Change the number 5 to the number 1 and save the file.
2- Shut down and power off the computer. Remove the old card and install the new card.
3- Power up the computer. It should automatically go to text mode login.
4- Log in as root. Copy your original XF86Config-r to XF86Config-4.original. Edit your /etc/X11/XF86Config-4 file to use the driver for the new card. That will only require you to change the driver section of this file. Here is the driver section of my XF86Config-4 file using an ATI Radeon 8500.
Identifier "ATI Technologies Inc R200 BB [Radeon All in Wonder 8500DV]"
Note that this uses the generic ati driver, not the one from ATI that provides OpenGL and acceleration.
5- Test your settings by running startx at the command prompt. If you have problems you can run /usr/X11R6/bin/sax2 to set your video settings.
6- Once you are satisfied that the new card will work then change your current directory to /etc and copy the inittab.original to inittab. This will restore your original run level settings.
Now you may want to consider installing the official drivers from ATI. There are numerous posts here in LQ where that question is asked and answered. Once you get the card working with the generic driver you can consider whether you want to use the official ATI drivers. I don't use them because I don't play games or use software that requires OpenGL. Not on this machine at least. I have a multimedia machine with an Nvdia card that uses Nvidia's OpenGL drivers, but that's another story.