Note: Always make a backup before messing with your system!
You should be able to do this with:
sudo apt-get install lubuntu-desktop
or
sudo apt-get install --no-install-recommends lubuntu-desktop
This should install lxde
.
At the login screen, you should have a settings wheel then next to the password input, where you can pick LXDE as desktop environment. If you login, your desktop environment should be the selected one. You can also log out and switch to another environment.
Wait for other replies, since I haven't done this before. I hope you get the idea.
Edit:
Misunderstood your question. Take a look at /home/your-user-name/
, this is where your files are stored.
There shouldn't be any files regarding any desktop environments, these are stored somewhere else. So your files will still be around, so you can install multiple other desktop environments to test out or work with with no affect on your personal files.