How to Migrate from accountKit to Firebase Phone sign up?

Question

I'm using AccountKit for phone sign up right now in our mobile app. As AccountKit will be deprecated soon so we are migrating with firebase phone sign up. I have set up a new user/Sign-up flow but still confused about old users when they log-in again using a new sign-in method(Firebase).

Current Login Process:

1. Client request with the access token.
2. Server request to account kit with an access token and get user data with UID
3. By this UID I check in current database user present or not.

Problem:

When old user tries to sign in through firebase then I'll get different UID(UID of firebase) which is not present in my database so it will consider it as a new user.

Backend: Node js

Can someone help me what would be the best approach to migrate?