Command To Change Default Display Manager (GDM, KDM, ...) in Ubuntu

There are some display manager available on Linux system. KDM, GDM, LXDM, lightDM is an example of display manager. Only one display manager run on one time where it bring user to be able to login through login screen, then enter to the desktop environment such KDE, GNOME, XFCE and many others.

It possible to change a default display manager using command line where the configuration will be changed automatically and will be loaded at boot time. For the Ubuntu operating system the following command is easy to do to switch a display manager.

Basic command : 
sudo dpkg-reconfigure display_manager

Example to run this program in command line :
When we run KDM then the command is :
sudo dpkg-reconfigure kdm

When we run GDM then the command is :
sudo dpkg-reconfigure gdm 

The blue screen with dialog will be appear and there a question and option about changing default display manager. See the following picture :