Você pode instalar a área de trabalho do Lubuntu e depois remover o Unity
Comando de uma linha:
sudo apt-get remove account-plugin-aim account-plugin-facebook account-plugin-flickr account-plugin-google account-plugin-icons account-plugin-identica account-plugin-jabber account-plugin-salut account-plugin-twitter account-plugin-windows-live account-plugin-yahoo acpi-support acpid activity-log-manager-common activity-log-manager-control-center adium-theme-ubuntu aisleriot apg app-install-data-partner appmenu-gtk appmenu-gtk3 appmenu-qt apt-xapian-index apturl apturl-common avahi-autoipd avahi-daemon bamfdaemon baobab binutils bluez-alsa bluez-cups bluez-gstreamer branding-ubuntu brasero brasero-cdrkit brasero-common brltty checkbox checkbox-qt compiz compiz-core compiz-gnome compiz-plugins-default cryptsetup-bin cups-bsd dc dconf-tools deja-dup doc-base duplicity dvd+rw-tools empathy empathy-common eog espeak-data evolution-data-server evolution-data-server-common example-content firefox firefox-globalmenu firefox-gnome-support folks-common fonts-kacst fonts-kacst-one fonts-khmeros-core fonts-lao fonts-lklug-sinhala fonts-opensymbol fonts-sil-abyssinica fonts-sil-padauk fonts-takao-pgothic fonts-thai-tlwg fonts-tibetan-machine fonts-tlwg-garuda fonts-tlwg-kinnari fonts-tlwg-loma fonts-tlwg-mono fonts-tlwg-norasi fonts-tlwg-purisa fonts-tlwg-sawasdee fonts-tlwg-typewriter fonts-tlwg-typist fonts-tlwg-typo fonts-tlwg-umpush fonts-tlwg-waree foomatic-db-engine freerdp-x11 gcalctool gcc gcc-4.7 gedit gedit-common geoclue geoclue-ubuntu-geoip gir1.2-accounts-1.0 gir1.2-appindicator3-0.1 gir1.2-atspi-2.0 gir1.2-dbusmenu-glib-0.4 gir1.2-dee-1.0 gir1.2-gdata-0.0 gir1.2-gmenu-3.0 gir1.2-gnomebluetooth-1.0 gir1.2-gnomekeyring-1.0 gir1.2-goa-1.0 gir1.2-gst-plugins-base-0.10 gir1.2-gstreamer-0.10 gir1.2-gtksource-3.0 gir1.2-gudev-1.0 gir1.2-indicate-0.7 gir1.2-javascriptcoregtk-3.0 gir1.2-messagingmenu-1.0 gir1.2-peas-1.0 gir1.2-rb-3.0 gir1.2-signon-1.0 gir1.2-soup-2.4 gir1.2-syncmenu-0.1 gir1.2-totem-1.0 gir1.2-totem-plparser-1.0 gir1.2-ubuntuoneui-3.0 gir1.2-unity-5.0 gir1.2-webkit-3.0 gnome-accessibility-themes gnome-bluetooth gnome-contacts gnome-control-center gnome-control-center-data gnome-control-center-signon gnome-desktop3-data gnome-font-viewer gnome-games-data gnome-mahjongg gnome-media gnome-menus gnome-online-accounts gnome-orca gnome-power-manager gnome-screensaver gnome-screenshot gnome-session gnome-session-bin gnome-session-canberra gnome-session-common gnome-settings-daemon gnome-sudoku gnome-system-log gnome-system-monitor gnome-terminal gnome-terminal-data gnome-user-guide gnome-user-share gnomine growisofs gstreamer0.10-alsa gstreamer0.10-gconf gstreamer0.10-plugins-base-apps gstreamer0.10-pulseaudio gstreamer0.10-tools gstreamer0.10-x gtk2-engines-murrine guile-1.8-libs gvfs-bin gwibber gwibber-service gwibber-service-facebook gwibber-service-identica gwibber-service-twitter hplip hplip-data hwdata ibus-gtk3 ibus-pinyin ibus-pinyin-db-android ibus-pinyin-db-open-phrase ibus-table indicator-appmenu indicator-datetime indicator-messages indicator-power indicator-printers indicator-session indicator-sound intel-gpu-tools kerneloops-daemon landscape-client-ui-install laptop-detect libaccount-plugin-1.0-0 libaccounts-glib0 libaccounts-qt1 libart-2.0-2 libasound2-plugins libatk-adaptor libatk-adaptor-data libavahi-core7 libavahi-gobject0 libbamf3-0 libboost-date-time1.49.0 libbrasero-media3-1 libbrlapi0.5 libc-dev-bin libc6-dev libcamel-1.2-40 libcanberra-gtk-module libcanberra-gtk0 libcanberra-gtk3-0 libcanberra-gtk3-module libcanberra-pulse libcanberra0 libclutter-1.0-0 libclutter-1.0-common libclutter-gst-1.0-0 libclutter-gtk-1.0-0 libcmis-0.2-2 libcogl-common libcogl-pango0 libcogl9 libcompizconfig0 libcrypt-passwdmd5-perl libcryptsetup4 libcurl3-nss libdaemon0 libdbusmenu-qt2 libdecoration0 libdee-1.0-4 libdmapsharing-3.0-2 libdotconf1.0 libebackend-1.2-5 libebook-1.2-14 libecal-1.2-15 libedata-book-1.2-15 libedata-cal-1.2-18 libedataserver-1.2-17 libespeak1 libexempi3 libexiv2-12 libexttextcat-1.0-0 libexttextcat-data libfile-copy-recursive-perl libfolks-eds25 libfolks-telepathy25 libfolks25 libfreerdp-plugins-standard libfreerdp1 libgail-common libgail18 libgdata-common libgdata13 libgeoclue0 libgexiv2-1 libglew1.8 libglewmx1.8 libgmime-2.6-0 libgnome-control-center1 libgnome-desktop-3-4 libgnome-media-profiles-3.0-0 libgnome-menu-3-0 libgnome-menu2 libgnomekbd-common libgnomekbd8 libgoa-1.0-0 libgoa-1.0-common libgomp1 libgtkmm-3.0-1 libgtksourceview-3.0-0 libgtksourceview-3.0-common libgtkspell-3-0 libgweather-3-1 libgweather-common libgwibber-gtk3 libgwibber3 libhpmud0 libhyphen0 libical0 libido3-0.1-0 libindicate5 libitm1 libjavascriptcoregtk-3.0-0 libjs-jquery liblcms1 liblouis-data liblouis2 liblua5.1-0 liblvm2app2.2 libmessaging-menu0 libmetacity-private0a libmission-control-plugins0 libmng1 libmusicbrainz5-0 libmx-1.0-2 libmx-bin libmx-common libmysqlclient18 libmythes-1.2-0 libnotify-bin libnss-mdns libnux-3.0-0 libnux-3.0-common liboauth0 libopencc1 libpackagekit-glib2-14 libpam-freerdp libpam-gnome-keyring libpeas-1.0-0 libpeas-common libprotobuf7 libprotoc7 libproxy1-plugin-gsettings libproxy1-plugin-networkmanager libpulsedsp libpython3.2 libqjson0 libqt4-dbus libqt4-declarative libqt4-designer libqt4-help libqt4-network libqt4-script libqt4-scripttools libqt4-sql libqt4-sql-mysql libqt4-sql-sqlite libqt4-svg libqt4-test libqt4-xml libqt4-xmlpatterns libqtassistantclient4 libqtcore4 libqtgui4 libqtwebkit4 libquadmath0 libquvi-scripts libquvi7 libraw5 libreoffice-base-core libreoffice-calc libreoffice-common libreoffice-core libreoffice-draw libreoffice-emailmerge libreoffice-gnome libreoffice-gtk libreoffice-help-en-us libreoffice-impress libreoffice-math libreoffice-ogltrans libreoffice-pdfimport libreoffice-presentation-minimizer libreoffice-presenter-console libreoffice-style-human libreoffice-style-tango libreoffice-writer librest-0.7-0 librhythmbox-core6 librsync1 libsane-hpaio libsensors4 libsgutils2-2 libsignon-extension1 libsignon-glib1 libsignon-plugins-common1 libsignon-qt1 libsnmp-base libsnmp15 libsonic0 libspeechd2 libspeexdsp1 libssh-4 libstlport4.6ldbl libsync-menu1 libsyncdaemon-1.0-1 libtelepathy-farstream2 libtelepathy-logger2 libtimezonemap1 libtotem-plparser17 libtotem0 libubuntuoneui-3.0-1 libufe-xidgetter0 libunity-core-6.0-5 libunity-misc4 libunity-protocol-private0 libunity-webapps0 libunity9 libuuid-perl libvncserver0 libwacom-common libwacom2 libwebkitgtk-3.0-0 libwebkitgtk-3.0-common libwmf0.2-7-gtk libyaml-tiny-perl libyelp0 libzeitgeist-1.0-1 light-themes lightdm-remote-session-freerdp lightdm-remote-session-uccsconfigure linux-headers-generic-pae linux-libc-dev make manpages-dev mcp-account-manager-uoa media-player-info metacity-common mousetweaks mscompress mtools mysql-common nautilus nautilus-sendto nautilus-sendto-empathy nautilus-share network-manager-pptp network-manager-pptp-gnome notify-osd notify-osd-icons nux-tools obexd-client onboard oneconf overlay-scrollbar overlay-scrollbar-gtk2 overlay-scrollbar-gtk3 pcmciautils pinyin-database pkg-config plymouth-theme-ubuntu-logo policykit-desktop-privileges pptp-linux printer-driver-c2esp printer-driver-foo2zjs printer-driver-hpcups printer-driver-min12xxw printer-driver-postscript-hp printer-driver-ptouch printer-driver-pxljr printer-driver-sag-gdi printer-driver-splix protobuf-compiler pulseaudio pulseaudio-module-bluetooth pulseaudio-module-gconf pulseaudio-module-x11 pulseaudio-utils python-apport python-configglue python-debtagshw python-dirspec python-gconf python-gi-cairo python-gnupginterface python-gst0.10 python-httplib2 python-imaging python-lxml python-mako python-markupsafe python-oauth python-openssl python-pam python-pexpect python-piston-mini-client python-problem-report python-protobuf python-pyinotify python-qt4 python-qt4-dbus python-renderpm python-reportlab python-reportlab-accel python-serial python-simplejson python-sip python-twisted-bin python-twisted-core python-twisted-names python-twisted-web python-ubuntu-sso-client python-ubuntuone-client python-ubuntuone-control-panel python-ubuntuone-storageprotocol python-uno python-xapian python-zeitgeist python-zope.interface python3-aptdaemon.pkcompat python3-brlapi python3-cairo python3-crypto python3-gi-cairo python3-httplib2 python3-louis python3-lxml python3-oauthlib python3-pyatspi2 python3-pycurl python3-speechd python3-virtkey qdbus qt-at-spi radeontool remmina remmina-common remmina-plugin-rdp remmina-plugin-vnc remote-login-service rhythmbox rhythmbox-data rhythmbox-mozilla rhythmbox-plugin-cdrecorder rhythmbox-plugin-magnatune rhythmbox-plugin-zeitgeist rhythmbox-plugins rhythmbox-ubuntuone rtkit samba-common samba-common-bin sane-utils seahorse session-migration sessioninstaller sgml-base shotwell signon-keyring-extension signon-plugin-oauth2 signon-plugin-password signon-ui signond sni-qt software-center software-center-aptdaemon-plugins sound-theme-freedesktop speech-dispatcher ssh-askpass-gnome syslinux syslinux-common syslinux-legacy telepathy-gabble telepathy-haze telepathy-idle telepathy-indicator telepathy-logger telepathy-mission-control-5 telepathy-salut thin-client-config-agent thunderbird thunderbird-globalmenu thunderbird-gnome-support toshset totem totem-common totem-mozilla totem-plugins ttf-indic-fonts-core ttf-punjabi-fonts ubuntu-artwork ubuntu-desktop ubuntu-docs ubuntu-mono ubuntu-settings ubuntu-sounds ubuntu-sso-client ubuntu-sso-client-qt ubuntu-system-service ubuntu-wallpapers ubuntu-wallpapers-quantal ubuntuone-client ubuntuone-client-gnome ubuntuone-control-panel ubuntuone-control-panel-qt ubuntuone-couch udisks unity unity-asset-pool unity-common unity-greeter unity-lens-applications unity-lens-files unity-lens-gwibber unity-lens-music unity-lens-photos unity-lens-shopping unity-lens-video unity-scope-gdocs unity-scope-musicstores unity-scope-video-remote unity-services unity-webapps-common unity-webapps-service uno-libs3 update-inetd ure usb-creator-common usb-creator-gtk vino wodim xcursor-themes xdiagnose xfonts-mathml xul-ext-ubufox xul-ext-unity xul-ext-websites-integration yelp yelp-xsl zeitgeist zeitgeist-core zeitgeist-datahub zenity zenity-common && sudo apt-get install lubuntu-desktop ubuntu-minimal && sudo /usr/lib/lightdm/lightdm-set-defaults -g lightdm-gtk-greeter
Fonte - voltando ao Ubuntu puro