For rooted devices only!

This app will let you conveniently switch out the emoji in your system. Currently available free emoji are Google (stock), Samsung, LG, and iOS. For a small charge, Twitter and Emoji One are also available.

Thanks to gonsa from XDA for providing the font files: forum.xda-developers.com/showthread.php?t=2618523

Busybox is recommended, and the app is untested without it.
All versions
October 25, 2016