To install Xubuntu after you have install Ubuntu 18.10 with the default Gnome desktop, make sure to take a backup of your data and then open a terminal window and run the following commands:
$ sudo apt-get install xubuntu-desktop <enter>
$ sudo apt-get purge whoopsie orage gigolo catfish snap* parole* <enter>
$ sudo apt-get purge ubuntu-gnome-desktop <enter>
$ sudo apt-get remove deja-dup nautilus nautilus-* gnome-power-manager gnome-screensaver gnome-termina* gnome-pane* gnome-applet* gnome-bluetooth gnome-desktop* gnome-sessio* gnome-user* gnome-shell-common zeitgeist-core libzeitgeist* gnome-control-center gnome-screenshot gnome* && sudo apt-get autoremove <enter>
$ sudo apt-get autoremove <enter>
$ sudo apt-get install --reinstall lightdm <enter>
$ sudo apt-get install glabels <enter>
$ sudo apt-get purge gedit aisleriot gnome-control-center gnome-settings <enter>
Choose lightdm as the default instead of gdm, if asked.
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