THIS! I want to know!
And, I've noticed an improvement in the performance of Light Flow, which makes me happy. The LED rotation is less random...
To replace the stock texting app in the lock screen with a secondary app that has been installed:
Tap Menu,
Tap System Settings
Scroll down to "Apps"
At the top, swipe right to show "All"
Scroll down to find Text Messaging
Tap on it
Tap Disable!
Now, when you go to the lock screen, your secondary app will show in its place!
Once it's disabled, don't freak out if you can't find it. It's at the very bottom of the "All" apps list!