Identificar que os pacotes precisam ser removidos de um meta-pacote como ubuntu-desktop é difícil. Mas você pode se livrar da maioria removendo estes pacotes
sudo apt remove compiz compiz-core compiz compiz-core compiz-gnome compiz-plugins-default evolution-data-server evolution-data-server-online-accounts file-roller gedit gedit-common gnome-accessibility-themes gnome-bluetooth gnome-calculator gnome-calendar gnome-desktop3-data gnome-disk-utility gnome-font-viewer gnome-keyring gnome-mahjongg gnome-menus gnome-mines gnome-orca gnome-power-manager gnome-screensaver gnome-screenshot gnome-session-bin gnome-session-canberra gnome-session-common gnome-settings-daemon-schemas gnome-software gnome-software-common gnome-sudoku gnome-system-log gnome-system-monitor gnome-terminal gnome-terminal-data gnome-user-guide gnome-user-share gnome-video-effects gparted gtk2-engines-murrine indicator-application indicator-appmenu indicator-bluetooth indicator-datetime indicator-keyboard indicator-messages indicator-power indicator-printers indicator-session indicator-sound libunity-action-qt1 libunity-control-center1 libunity-core-6.0-9 libunity-gtk2-parser0 libunity-gtk3-parser0 libunity-misc4 libunity-protocol-private0 libunity-scopes-json-def-desktop libunity-settings-daemon1 libunity-webapps0 libunity9 light-themes lightdm metacity metacity-common nautilus nautilus-data nautilus-sendto nautilus-share network-manager-gnome network-manager-pptp-gnome notify-osd notify-osd-icons overlay-scrollbar overlay-scrollbar-gtk2 remmina remmina-common remmina-plugin-rdp remmina-plugin-vnc rhythmbox rhythmbox-data rhythmbox-plugin-zeitgeist rhythmbox-plugins shotwell shotwell-common thunderbird thunderbird-gnome-support totem totem-common totem-plugins transmission-common transmission-gtk ubuntu-artwork ubuntu-desktop ubuntu-docs ubuntu-mobile-icons ubuntu-session ubuntu-settings ubuntu-software ubuntu-sounds ubuntu-system-service ubuntu-touch-sounds ubuntu-ui-toolkit-theme ubuntu-wallpapers ubuntu-wallpapers-xenial unity unity-accessibility-profiles unity-asset-pool unity-control-center unity-control-center-faces unity-control-center-signon unity-greeter unity-gtk-module-common unity-gtk2-module unity-gtk3-module unity-lens-applications unity-lens-files unity-lens-music unity-lens-photos unity-lens-video unity-schemas unity-scope-calculator unity-scope-chromiumbookmarks unity-scope-colourlovers unity-scope-devhelp unity-scope-firefoxbookmarks unity-scope-gdrive unity-scope-home unity-scope-manpages unity-scope-openclipart unity-scope-texdoc unity-scope-tomboy unity-scope-video-remote unity-scope-virtualbox unity-scope-yelp unity-scope-zotero unity-scopes-master-default unity-scopes-runner unity-services unity-settings-daemon unity-webapps-common unity-webapps-qml unity-webapps-service zeitgeist-core zeitgeist-datahub
Após este comando, use
sudo apt install kubuntu-desktop
Ele irá extrair pacotes de kubuntu-desktop se algo for removido pelo comando anterior.