migration.py

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations, models

def update_plans(apps, schema_editor):
    # free to Free
    for user in User.objects.filter(plan='free'):
      user.plan = 'Free'
      user.save()
    
    # premium to Monthly
    for user in User.objects.filter(plan='premium'):
      user.plan = 'Monthly'
      user.save()
      
    # default to OnDemand
    for user in User.objects.filter(plan='default'):
      user.plan = 'OnDemand'
      user.save()

def noop(apps, schema_editor):
    pass

# manage.py makemigrations --empty user
class Migration(migrations.Migration):
    # ...

    operations = [
        migrations.RunPython(update_plans, noop),
    ]