Missing DEFAULT_DM in /etc/sysconfig/displaymanager

Using Tumbleweed (VERSION_ID=“20190301”) and I want to switch from SDDM to LightDM. I cannot find any site that explains how to switch DM and the only way I know how is to change it in /etc/sysconfig/displaymanager (I usually use YaST, but have been known to make manual changes, too). This is what is in my displaymanager file starting from the first line:

## Path:        Desktop/Display manager
## Type:        string(Xorg)
## Default:     "Xorg"
#
DISPLAYMANAGER_XSERVER="Xorg"
## Path:        Desktop/Display manager
## Description: settings to generate a proper displaymanager config
## Type:        string(kdm,xdm,gdm,wdm,entrance,console,lightdm,sddm)
## Default:     ""
#
# Here you can set the default Display manager (kdm/xdm/gdm/wdm/entrance/console).
# all changes in this file require a restart of the displaymanager
#

## Path:        Desktop/Display manager
## Description: settings to generate a proper displaymanager config
## Type:        yesno
## Default:     no
#
# Allow remote access (XDMCP) to your display manager (xdm/kdm/gdm). Please note
# that a modified kdm or xdm configuration, e.g. by KDE control center
# will not be changed. For gdm, values will be updated after change.
# XDMCP service should run only on trusted networks and you have to disable
# firewall for interfaces, where you want to provide this service.
#
DISPLAYMANAGER_REMOTE_ACCESS="no"


To me, it looks like I am missing DEFAULT_DM. Is it okay to just add it? Or has switching DM changed? Thanks for any help, or for any sites to which you can direct me!

https://en.opensuse.org/SDB:Change_Display_Manager#Tumbleweed.2FLeap_15_and_later

Yeah, that doesn’t change it, only selects it. Mine says lightdm but sddm is still the DM.
And SDDM is not even a choice in the alternatives list, only lightdm and console.

After rebooting or restarting X?

And SDDM is not even a choice in the alternatives list, only lightdm and console.
That does seem wrong. What is actually installed? Using zypper to ensure LightDM is installed and SDDM is not installed ought to fix it. If it doesn’t, try force reinstalling LightDM.

Please share the output from

zypper se -si dm
sudo update-alternatives --list default-displaymanager