To change the display manager (the login screen) from one kind to another (like LightDM to KDM), here is a simple trick. With Root privileges, execute "dpkg-reconfigure CHOICE" where "CHOICE" is the package name of the desired display manager that will be the new default. For instance, if LightDM and MDM are installed, but MDM is the default, then run "dpkg-reconfigure lightdm" to make the new default LightDM.
If the display manager ever has problems or bugs, install a different display manager and make it the default. Alternatively, re-install the display manager. For instance, to fix the common LightDM bugs, run "apt-get install lightdm-gtk-greeter".