I did something similar to this a while back. I had a dual boot box with Windows 2000/Mandriva. Mandriva was my first Linux distro, however, through others I learned that it wasn't the most "versital" and decided to switch to Fedora Core 5. Like uselpa said, just assign the instillation to the same partition SuSE was on and Ubuntu should just overwrite it with no problems.
...I agree though...don't do anything without backing up first!