When AutoCAD is uninstalled, not everything is removed from the system. Registry settings, for example, remain in place. This useful article describes all the things you should do to completely remove Autodesk products from your system.
Bit late now, I know, but it's worth pointing out that you can run different versions of AutoCAD side-by-side on the same PC. I always install a new version alongside the old one in order to make a graceful transition; that way I can always drop back to the previous one if necessary. Only when I'm happy with the new version do I uninstall the old.