You can install Lubuntu onto any installed version of Ubuntu. This will give you the option on boot, or logging out to choose which system to use.

For Ubuntu 11.10 and later

Launch your Terminal

sudo apt-get update
sudo apt-get install lubuntu-desktop

For Ubuntu 10.04

Launch your Terminal

sudo add-apt-repository ppa:lubuntu-desktop/ppa
sudo apt-get update
sudo apt-get install --no-install-recommends lubuntu-desktop

Removing other x/k/ubuntu components after upgrading to Lubuntu

Adding Lubuntu to your system will keep all the applications from your existing system. If you wish to get rid of them Pure LXDE has the details - make sure you use the correct version of the instructions. Prior to 11.10 you need simply alter the very end of the command from:

sudo apt-get install lubuntu-desktop

To:

sudo apt-get install --no-install-recommends lubuntu-desktop

This will not delete any of your own data from those applications. If you later wish to re-add them, your data will be there waiting for you