I would like to know how to convert Ubuntu into Lubuntu, without a fresh installation of the system.
You can install lubuntu-desktop, which is the desktop environment lxde plus the applications that make up the lubuntu distribution by typing in a terminal:
sudo apt-get update sudo apt-get install lubuntu-desktop
Reboot for effect. All your files should still be there;)
I wouldn't bother removing Gnome completely unless you are very low on disk space.
It's also possible to install Lubuntu-desktop from the ubuntu software center.
You can unistall current unity desktop environment and install LXDE desktop environment.
How to remove GNOME completely? will help you to remove your current desktop environment.
Then type as follow to install lxde: sudo apt-get install lxde
This will give you desktop environment similar to that of lubuntu.
for more information, see http://wiki.lxde.org/en/Ubuntu