If the provider is worth its salt¹ it will prefix the stored password with the hash that was used. That way it can update the hash when the user next logs in if it is out of date.
E.g.: {argon2}…
¹ See what I did there?
E.g.: {argon2}…
¹ See what I did there?
I still don't know how this would help with a migration, though. They would still need to run both auth systems in parallel until every single user has logged in again - or force everyone to create a new password. Right?
But in reality, I doubt they would release either of those things.