Reconfigure the display-manager non-interactively

I am trying to change the default display-manager on a Ubuntu 18.04.1 in a non-interactive way to do this in a script.

Everything works as expected when using

• dpkg-reconfigure gdm3
• dpkg-reconfigure lightdm

I tried setting shared/default-x-display-manager to a valid display-manager and run dpkg-reconfigure, but that did not do the trick.

First check which display-manager is set.

root@host:~# cat /etc/X11/default-display-manager/usr/sbin/gdm3root@host:~# cat /etc/X11/default-display-manager/usr/sbin/gdm3

Then set shared/default-x-display-manager to lightdm and validate it.

root@host:~# echo set shared/default-x-display-manager lightdm | debconf-communicate 0 value setroot@host:~# echo get shared/default-x-display-manager | debconf-communicate 0 lightdm

Finally run dpkg-reconfigure --frontend noninteractive lightdm and check which display-manager is active. Which unfortunately is gdm3 again.

root@host:~# dpkg-reconfigure --frontend noninteractive lightdmroot@host:~# echo get shared/default-x-display-manager | debconf-communicate 0 gdm3root@host:~# cat /etc/X11/default-display-manager /usr/sbin/gdm3root@host:~# ll /etc/systemd/system/display-manager.servicelrwxrwxrwx 1 root root 32 Jan 31 20:03 /etc/systemd/system/display-manager.service -> /lib/systemd/system/gdm3.service

The same happens if I want to switch from lightdm to gdm3.
Anything I am missing here or is this not intended to work at all?

I have seen in the scripts for gdm3 that if there is a /etc/X11/default-display-manager.debconf-update file, the systemd unit and the /etc/X11/default-display-manager are updated accordingly.
Unfortunately this does not work for lightdm as this one uses some other method(s) to do the work.

Hope someone else can shed some light on this and knows a convenient way to configure the display-manager non-interactively.