1. A clean install
A clean install is the simplest way to do it, and then to add whatever preferences you like to have.
2. Change your system
Backup your system (or at least your /home folder or partition. Then install lubuntu-desktop into your present system
and make it Pure Lubuntu according to these links
sudo apt-get install lubuntu-desktop
Edit: But remember that Lubuntu has no long time support. Its end of life is October 2013, so think twice before doing this. Maybe it is better to run Ubuntu Server with the addition of XFCE
and enjoy the LTS support of packages belonging to Xubuntu until April 2015.
sudo apt-get install xfce4
You can also install
and make Pure Xubuntu
sudo apt-get install xubuntu-desktop