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.

Launch your terminal and copy the commands to the command line one at a time:

sudo apt update
sudo apt install lubuntu-desktop

or

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

if you want a lighter installation.

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.

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.