Comment 27 for bug 921384

Yes, I have always read that you should purge the driver first before installing a newer version. You can easily remove fglrx with this command- sudo apt-get remove --purge fglrx fglrx_* fglrx-amdcccle* fglrx-dev*
After you run this install the new driver then reboot and hopefully you'll boot into lightdm again. If you need any help let me know and I'll try to help.