Other apps that only want to read new messages can instead receive the SMS_RECEIVED_ACTION broadcast intent when a new SMS arrives. However, only the app that receives the SMS_DELIVER_ACTION broadcast (the user-specified default SMS app) is able to write to the SMS Provider defined by the android.provider.Telephony class and subclasses.
As such, it's important that you update your messaging app as soon as possible to be available as a default SMS app, because although your existing app won't crash on an Android 4.4 device, it will silently fail when attempting to write to the SMS Provider.