If your push notification is setup with Google Cloud messaging and its working, than for the new androapp apk (version 6.0.4 onwards) you have to migrate your existing GCM app to firebase, so that your existing customers remains intact.
Follow these steps to migrate
- go to https://console.firebase.google.com/
- Click on or import a Google Project
- Select the relevant project from the list
- And click on Add Firebase
- Now continue from step 4 from this article