Ubuntu used to come with LightDM, and Gnome desktop as default.
Ubuntu with a different DM and GUI was given a prefix (Kubutu, Lubuntu, Xubuntu, Edubuntu).
When Ubuntu switched to Unity DM and Desktop, the phrase Ubuntu Gnome was coined to refer to the old installation while Ubuntu referred to the new.
With the release of 17.04, Ubuntu has gone to a new DM (gnome-shell) and returned to the gnome desktop (named ubuntu-desktop).
Removing one DM and desktop and installing another is no different than installing the flavor of Ubuntu that originally has that DM and desktop.
For example, if you remove the Unity DM and desktop then install KDM and Plasma/KDE desktop, you would now have Kubuntu. Note:in most cases the greeter (login) has to changed with the DM.