Q:

Can I use Firebase as a PassThrogh alone for IOS push notification

Can I use Firebase push notification as a pass through for sending notification to IOS devices by registering IOS credentials in the Firebase ?  Becasue I am already having the IOS app released and having the device tokens in my backend.  
Is it necessary that i have to change my IOS app in order to leverage FCM ?  but in Azure Notification Hub I dont have to really change the IOS app i can use it as a pass through but it has some cost involved. 

A:

FCM uses its own device tokens to identify devices for push notifications for both Android and iOS devices.   It doesn't accept OS-specific tokens.   If you want to use FCM, you will definitely have to make changes to your app in order for it to be effective.   The documentation should be clear on what you have to do to integrate it.