If your changes break your old schema this won't work - in which case tools like south or django evolution are great.
+
+Add a new model to the DB
+
+$python manage.py schemamigration <your_app> --auto
+$python manage.py migrate
+
======== update django database to reflect changes in existing models with migration system (e.g., south) =========
As south is already installed , you just have to do: