Alterando atributos de certos fields da classe Pai

models.py

class User(models.Model):
    """
    Users within the Django authentication system are represented by this
    model.

    Username and password are required. Other fields are optional.
    """
    username = models.CharField(_('username'), max_length=30, unique=True,
        help_text=_('Required. 30 characters or fewer. Letters, numbers and '
                    '@/./+/-/_ characters'))
    first_name = models.CharField(_('first name'), max_length=30, blank=True)
    last_name = models.CharField(_('last name'), max_length=30, blank=True)
    email = models.EmailField(_('e-mail address'), blank=True)
    password = models.CharField(_('password'), max_length=128)
    is_staff = models.BooleanField(_('staff status'), default=False,
        help_text=_('Designates whether the user can log into this admin '
                    'site.'))
    is_active = models.BooleanField(_('active'), default=True,
        help_text=_('Designates whether this user should be treated as '
                    'active. Unselect this instead of deleting accounts.'))
    is_superuser = models.BooleanField(_('superuser status'), default=False,
        help_text=_('Designates that this user has all permissions without '
                    'explicitly assigning them.'))
    last_login = models.DateTimeField(_('last login'), default=timezone.now)
    date_joined = models.DateTimeField(_('date joined'), default=timezone.now)
    groups = models.ManyToManyField(Group, verbose_name=_('groups'),
        blank=True, help_text=_('The groups this user belongs to. A user will '
                                'get all permissions granted to each of '
                                'his/her group.'))
    user_permissions = models.ManyToManyField(Permission,
        verbose_name=_('user permissions'), blank=True,
        help_text='Specific permissions for this user.')
    objects = UserManager()

    class Meta:
        verbose_name = _('user')
        verbose_name_plural = _('users')

    # ...

models_extended.py

# -*- coding: utf8 -*-

from django.contrib.auth.models import User


class Other(User):
    """Fonte:
        http://stackoverflow.com/questions/4904230/django-change-default-value-for-an-extended-model-class
    """
    # other fields

    def __init__(self, *args, **kwargs):
        self._meta.get_fields('username').max_length=100
        super(Other, self).__init(*args, **kwargs)