Migrate devise 2.0 users to devise 3.0

I am currently migrating a Rails 3 SQL database to a Rails 4 application. We copied the fields like encrypted_password, password_salt, etc. But I'm struggling in finding out, how can I use the valid_password? method for both old 2.0 and 3.0 devise users version, since it appears to have changed the way they handle passwords. Thanks in advance.