from django.db import migrations

from project.groups import Group

def fill_user_group(apps, schema_editor):
    User = apps.get_model('project', 'User')
    User.objects.filter(type='Visitor').update(group=Group.VISITOR)
    # or you could find a single one and just .save()

class Migration(migrations.Migration):

    dependencies = []

    # what to run on migration, and what to run on rollback (noop)
    operations = [
        migrations.RunPython(fill_user_group, migrations.RunPython.noop),
    ]

Sources

• Django Docs - Migration Operations