Changing Display Manager

Changing between KDM and GDM is made safe and easy with the help of the dpkg-reconfigure. Simply follow the procedure below to change between display managers.
From the Desktop start a shell session.
At the prompt enter the following command
sudo dpkg-reconfigure kdm

When prompted enter your password. This will start the Ubuntu Configuration manager. You will remember this screen from the installation, so select OK.
Because there are now two Display Managers installed and only one display manager can be used to manage a given X Server, you will be prompted to select a default. Selected either GDM or KDM then OK.
Shutdown and Restart the computer. When the system boots it will run the display manager of your choice.

Source: http://www.kubuntu.org/documentation.php#d0e421

However .... sudo dpkg-reconfigure kdm returns:

* Reloading K Display Manager configuration... [ OK ]

... followed by a deathly silence.