Make KDM the default display manager

I'm giving KDE another try - so far so good. I installed everything using apt-get install kubuntu-desktop. During the installation it asked me which login manager to use and I chose LightDM - however, now I'm finding user sessions (KDE) are behaving a little funny - when left for some time, they lock out asking for password, but there's no way to switch user (i.e. there's a small window asking for password with Ok and Cancel buttons - no Switch User, Shutdown, Suspend or anything).

Pressing Ctrl-Alt-F[something] shows a text log, where I can see a message "Not starting KDM because it's not the default manager" - so I think my problem can be fixed by making it default.

How can I do this?

(actually, trying to switch user via K-menu - Leave - Switch user results in the same dialog: "The session was locked by sergey, please enter your password")

Reconfiguring

With the Debian and the Debian based (K/X/L/ubuntu) you could use the dpkg-reconfigure.

    man dpkg-reconfigure:

    NAME
           dpkg-reconfigure - reconfigure an already installed package

    SYNOPSIS
            dpkg-reconfigure [options] packages

    DESCRIPTION
    dpkg-reconfigure reconfigures packages after they have already been 
    installed. Pass it the names of a package or packages to reconfigure. 
    It will ask configuration questions, much like when the package was first installed...

That is:

sudo dpkg-reconfigure kdm

or

sudo dpkg-reconfigure gdm

or ...

Kubuntu & LightDM

The LightDM is : https://wiki.kubuntu.org/LightDM

There was a plan: https://blueprints.launchpad.net/ubuntu/+spec/desktop-o-kubuntu-lightdm

QLightDM: https://lists.ubuntu.com/archives/kubuntu-devel/2011-June/005248.html

With the Oneiric the Kubuntu desktop is depending the KDM: http://packages.ubuntu.com/oneiric/kubuntu-desktop

please have a look at the following file:

 /etc/X11/default-display-manager

There you can change to kdm.

Solution that doesn't require root access

Make a file ~/.xinitrc:

# makes a kde a default desktop after `startx':
exec startkde

Also make ~/.xsession:

# makes a kde a default desktop after `xdm':
exec startkde