You getting error, because your changes must override old models and migrations. To do that
$ tutor local run lms bash
To delete old migrations
$ ./manage.py lms migrate custom_reg_form zero
$ tutor images build openedx
$ tutor local quickstart # you should see new migrations, if not ? just run
$ tutor local run lms bash
$ ./manage.py lms makemigrations
$ ./manage.py lms migrate