It sounds like Ubuntu resorted to the generic vesa driver. If this is the case you would see "vesa" as the driver in the Device section of your /etc/X11/xorg.conf file, in which case you would edit that part of the file and replace "vesa" with "ati".
If that seems like a challenge, try running this command:
sudo dpkg-reconfigure xserver-xorg
This command in Debian runs a more friendly graphic application for reconfiguring the xorg.conf file. It should do the same for Ubuntu. Although I'm not entirely sure, but I believe it's the same utility that was used during the initial installation to configure the /etc/X11/xorg.conf file, and if you choose to leave all answers set at the default it presents, you should have the same graphics you had from a fresh install.
It is advisable to first gather all kinds of information from manuals either in your possession or on-line to get proper refresh rates for the monitor and the likes to really customize this file, for questions you don't know the answer to, just hit enter to accept the default.