Other desktop enviroments downloaded&installed from the repos or Ubuntu Software Center can and will overwrite data to a file:
/etc/lightdm/lighdm.conf and/or /etc/lightdm/lightdm.conf.d/50-unity-greeter.conf
The new lightdm.conf file will look like this after installing KDE Plasma Desktop:
Code:
[SeatDefaults]
user-session=kde-plasma
greeter-session=lightdm-kde-greeter
To restore your original greeter you have to edit : /etc/lightdm/lightdm.conf in this way:
Code:
[SeatDefaults]
user-session=unity-greeter
type=unity
regards..