Migrate my website from GCM to FCM

My website uses GCM (via OneSignal) push notifications system. I have to migrate push notification system to FCM (Firebase). My goal is to migrate old tokens (and theirs relative tags/preferences) stored onto OneSignal into mine DB (on MongoDB) and send to that tokens my push notification via Firebase.

Reading Firebase's doc I don't understand if it is possible and how it is possible to reuse (or migrate/translate) old OneSignal tokens with Firebase.

Then:

1. Is it possible to reuse old OneSignal token with Firebase?
2. If n°1 is FALSE, how can I migrate old token to new token accessible with Firebase? Is there an API or JS method defined into Firebase SDK in order to translate old tokens.