Migration of old user data from Realm to new Realm Mongodb

I have checked the migration guide and have made changes accordingly on the app side code.
Other than that how do we migrate the old user date to new, since he will not be having his auth credentials in realm, how do handle them?
User data is migrated and is present in the atlas db.
When old user tries to login it will fail, because that user in not present in sync.
Forecefully register that user in auth, and call login again ?
Apart from this any other thing which i should be aware of ?