Why did my lock screen change?

I had the same problem. For some reasons, my "Screen Reader" option in "Universal Access" was turned on (I suspect my daughter) and was preventing the new lock screen to kick in.

Apparently, "on-screen keyboard" option could also cause this behaviour as suggested in this thread:

How to restore the new 14.04-lockscreen

I suggest you check in System Settings > Universal Access to see if you have any option enabled that could be the cause of the problem.

Good luck

Both the lock screen exist in your system. The first one is provided by Lightdm, you can try

dm-tool lock

The second one is provided by gnome, use

gnome-screensaver-command -l

To set dm-tool as your desired lock screen install, compizconfig-settings-manager

sudo apt-get install compizconfig-settings-manager

Go to CompizConfig Settings Manager, click on Ubuntu Unity Plugin and on the General tab, under Lockscreen, you can select LightDM lockscreen.

webupd8 described how to switch between unity and lightdm lock screen.