Why doesn't the welcome screen change when switching between DEs?

Question

linux

Ubuntu 16.04.01 LTS
The LXDE environment has been installed. the welcome screen has changed to match.
Now I can change the environment to Unity or LXDE at will, but the welcome screen remains LXDE.
How to bring back the Unity welcome screen?

Answer 1

The welcome screen shows DM - Display manager, a program completely independent of DE, just some DM usually comes with DE. There is also a standard DM - xdm, which comes with X. I have no idea which DM in Unity, but it is he who is responsible for the welcome screen.

Answer 2

sudo systemctl disable dm_which_now
sudo systemctl enable desired_dm
or you can try:
sudo dpkg-reconfigure lightdm