Migrating from auth_user in Django

I created a custom user model in Django and it worked up fine. However, I decided to create a custom model to suit my needs after the project was up and running.

As a result, I will need to migrate the schema (Currently, when I register a user, the code is still referencing to the the auth_user database tables where as the new custom user table is user.)

I have set the AUTH_USER_MODEL in settings.py to userapp.User, where userapp is my custom user app and User is the Model that inherits from the AbstractUser model.

I am fairly new to Django and cannot understand how to achieve this. One obvious way to clean install the database, which is not something that I'm looking to do as it will remove all my data.

How do I migrate then? I've heard South is used for that but I don't know how to use it. Besides I think South isn't required in the recent versions of Django.

My version of Django is 1.8.2.