How to Switch Between The New Unity Lockscreen And LightDM In Ubuntu 14.04

To switch between LightDM and Unity for screen locking, firstly install CompizConfig Settings Manager (CCSM):

sudo apt-get install compizconfig-settings-manager

Then open CompizConfig Settings Manager, click on Ubuntu Unity Plugin and on the General tab, under "Lockscreen", you can select to use Unity, LightDM or no lockscreen:

That's it - there's no need to restart the system or log out, the settings should be applied instantly.