Services and Subscriptions - duplicated services

Hi,

In my setup I have two services that are connected to Firebase Cloud Messaging for Android and Apple Push Notifications for iOS version of the app. The setup looks more-or-less like that:

Service1:

- for iOS is using token-based authentication, with key id, team, bundle id and private key

- for Android is using FCM HTTP (legacy) API version and project key

Service2:

- for iOS is using the same authentication details as Service1

- for Android is using FCM HTTP v1 API version with project id and private key

HTTP for FCM will be sunsetting soon, so I wanted to simplify the setup the keep only one service - especially that for iOS app both services are connecting to the same project. However, I have doubts before turning off Service1, because I see a different number of active iOS devices in Service1 and Servic2 - even though they are same app and should return the same users. 

What would be the best way to keep only one service and make sure that all the users (especially for iOS) are 'migrated' to the remaining service in Adobe Campaign?

How does actually the process of fetching data from FCM/APN work?

Thanks,

Dominik