If your device is rooted then you can now change your system emojis using Emoji Switch.

The app is now out of beta!
If you have problems and can't use the app for some reason then don't hesitate to contact me at: [email protected]

Want to switch to your old emojis?
No problem. You can now restore your old emojis using EmojiSwitch.

Currently supported emoji packs:
Twitter EmojiOne AOSP Emojis iPhone Emojis and more!