This should be as simple as burning the Lubuntu iso to your CD and going through the installation as normal. After you've gotten that installed, you should be able to just execute a few commands to remove the Lubuntu desktop and install the Ubuntu desktop in its place.
From the Lubuntu desktop, press Ctrl + Alt + F2 to get to tty/terminal mode.
sudo apt-get update
sudo apt-get install ubuntu-desktop
sudo apt-get remove lubuntu-desktop lubuntu-artwork plymouth-theme-lubuntu-logo plymouth-theme-lubuntu-text
sudo apt-get autoremove
This will possibly/probably leave some leftover Lubuntu packages (text editors, IM client, etc), so if you want to remove everything related to Lubuntu, the following command should work:
P.S. Command pulled from this post
EDIT: It looks like this page is the original source for the following code. Thanks to @TrailRider for the link. :)
sudo apt-get remove abiword abiword-common abiword-plugin-grammar abiword-plugin-mathview ace-of-penguins audacious audacious-plugins chromium-browser chromium-browser-l10n chromium-codecs-ffmpeg elementary-icon-theme esound-common galculator gdebi gdebi-core giblib1 gnome-icon-theme-full gnome-mplayer gnome-system-tools gnome-time-admin gnumeric gnumeric-common gnumeric-doc gpicview gtk2-engines-pixbuf guvcview hardinfo indicator-status-provider-pidgin leafpad libabiword-3.0 libaiksaurus-1.2-0c2a libaiksaurus-1.2-data libaiksaurusgtk-1.2-0c2a libaudclient2 libaudcore1 libaudiofile1 libavcodec53 libavformat53 libavutil51 libbinio1ldbl libcddb2 libcompfaceg1 libcue1 libdca0 libdirectfb-1.2-9 libdiscid0 libdvdnav4 libdvdread4 libenca0 libencode-locale-perl libept1.4.12 libesd0 libexo-1-0 libexo-common libfaad2 libfile-listing-perl libfluidsynth1 libfm-data libfm-gtk-data libfm-gtk3 libfm3 libfont-afm-perl libgdome2-0 libgdome2-cpp-smart0c2a libglade2-0 libgoffice-0.8-8 libgoffice-0.8-8-common libgringotts2 libgsf-1-114 libgsf-1-common libgsm1 libgtkmathview0c2a libhtml-form-perl libhtml-format-perl libhtml-parser-perl libhtml-tagset-perl libhtml-tree-perl libhttp-cookies-perl libhttp-daemon-perl libhttp-date-perl libhttp-message-perl libhttp-negotiate-perl libid3tag0 libimlib2 libio-socket-ssl-perl libjpeg-progs liblink-grammar4 libloudmouth1-0 liblwp-mediatypes-perl liblwp-protocol-https-perl libmailtools-perl libmcrypt4 libmcs1 libmenu-cache3 libmms0 libmowgli2 libmusicbrainz3-6 libnet-dbus-perl libnet-http-perl libnet-ssleay-perl libobrender29 libobt2 libonig2 liboobs-1-5 libopenal1 libopts25 libots0 libpisock9 libpolkit-gtk-1-0 libpostproc52 libresid-builder0c2a libschroedinger-1.0-0 libsidplay2 libsvga1 libswscale2 libtar0 libtie-ixhash-perl libtimedate-perl libts-0.0-0 libuniconf4.6 liburi-perl libva1 libvdpau1 libvpx1 libwebcam0 libwv-1.2-4 libwvstreams4.6-base libwvstreams4.6-extras libwww-perl libwww-robotrules-perl libxfce4ui-1-0 libxfce4util-bin libxfce4util-common libxfce4util4 libxfcegui4-4 libxfconf-0-2 libxml-parser-perl libxml-twig-perl libxml-xpath-perl libxss1 libxvidcore4 link-grammar-dictionaries-en lm-sensors lubuntu-artwork lubuntu-core lubuntu-default-settings lubuntu-desktop lubuntu-icon-theme lxappearance lxappearance-obconf lxde-common lxde-core lxdm lxinput lxkeymap lxlauncher lxmenu-data lxpanel lxpanel-indicator-applet-plugin lxrandr lxsession lxsession-edit lxshortcut lxtask lxterminal mplayer mtpaint ntp obconf openbox openbox-themes osmo pcmanfm pidgin pidgin-data pidgin-libnotify pidgin-microblog plymouth-theme-lubuntu-logo plymouth-theme-lubuntu-text scrot sylpheed sylpheed-doc sylpheed-i18n sylpheed-plugins synaptic system-tools-backends transmission tsconf ttf-lyx uvcdynctrl uvcdynctrl-data wvdial xfburn xfce-keyboard-shortcuts xfce4-power-manager xfce4-power-manager-data xfconf xfonts-100dpi xpad xscreensaver xscreensaver-data
If you still have the Lubuntu login screen, you should be able to fix that by doing this:
(source)
sudo apt-get install lightdm lightdm-greeter-example-gtk
sudo dpkg-reconfigure lightdm
ubuntu-desktop
is the best solution....if you want to remove Lubuntu after see psychocats.net/ubuntucat/tag/pure-ubuntu and use the command to shown to remove all Lubuntu's programs....psycocat's is a good site that has been referred to many times on AU, I have used some of her instructions myself....