If you’re not a fan of the default messaging app on Android, then in all likelihood you already use a third party SMS app, despite it using private APIs — which is frowned upon by Google. Now however, it’s about to get a lot easier and you’ll even be able to change your default messaging app in Android 4.4, with the company announcing that it will be making messaging APIs public.
When you download an SMS, or other messaging app from the Play Store you’ll be asked which one you want to make your default SMS app, once you’ve chosen which one you want, all your sent and received messages will be directed to the new app.
This does come with a rather large caveat though, with existing backup and restore apps being unable to restore SMS messages in Android 4.4, as that app will have to be set as the default app.
Source: Android Developers Blog