As Nash says it is better for legacy users that you let them login with their old method but if you cant.
You should invite your existing users to migrate to new form of login.
Explain them why the change.
Ask them to migrate their user.
Notify them the confirmation of the new way for login.
But as I say. The better is to let the two ways available.
Your options are:
Keep email and social login option for legacy users: Don't allow them for new users, though.
Notify old users via email or in the application of the migration: Tell the users that they need to change their login method until day X (This is also what a lot of banks have to do if they requirements for login change).
We've been experiencing some complains about some will mistype their email AND create a new account and not understand why they were not seeing their information in their User home.
I don't think you should let them create am account without validating the email (or at least you should have an internal differentiation between "good" emails and ...
Assuming you have a public website, there are following important points:
Sign up should be as easy as a button click and focus should be to introduce user to website features asap.
Allowing only Humans should be a priority.
You need to remember Users for further personalized communication(like feedback, offers etc.) even if they used the app one time. So ...