When you do a fresh installation, all additional programs and all files are gone. So if you do this, you should backup all your data. This results in a clean install.
But when you do an upgrade (via update manager or sudo apt-get dist-upgrade) all programs should be preserved. But it might result in an unstable install.
And another warning: if you have manually installed the driver for the Nvidia video card, uninstall it before you upgrade
Of course, the display will not work any more or doesn't work properly. There are ways to fix it if you are not so used to the command line:
sudo /etc/init.d/gdm stop
This should restore the display but results in a low resolution.
sudo cp -i /etc/X11/xorg.conf.failsave /etc/X11/xorg.conf
sudo /etc/init.d/gdm restart
But if you have installed the driver via restricted drivers manager, it should go well.