Android has always allowed the ability to replace any of the built-in apps with something else. But while it is possible to do this for certain functions using public APIs, for others, developers have to rely on private APIs, a practice Google condemns.
This is true for all the SMS applications currently on the store. But that is about to change with Android 4.4 KitKat. Google has announced that they will be making the APIs required for this public. Moreover, they are also introducing the concept of setting your default SMS app.
Once implemented correctly, you will get the option to set your new SMS app as the default messaging app on KitKat and all the sent and received messages will be directed to the new app. Google has provided all the required information for developers to update their existing SMS apps so that they work properly in KitKat in their blog post. If you’d like to read more about that, click on the source link below.