This [#https://developers.google.com/cloud-messaging/android/android-migrate-fcm] is guide on migration.
Basically, for migration we have to make changes at 3 places,
1) Firebase Console Side
2) Server Side
3) Client Side (App Side) - This change I have made.
I need the updated google-services.json file and the Legacy Server Key (for testing FCM Notification) which we can get from the Firebase Console after Migration is done on the Firebase Console Side.