http://chriskief.com/2013/09/28/django-modelform-and-conditionally-disabled-readonly-fields/

forms.py

# the following fields can only be updated when the is_disabled property is set to false
def clean_field_1(self):
    if self.instance.is_disabled:
        return self.instance.field_1
    else:
        return self.cleaned_data.get('field_1')

def clean_field_2(self):
    if self.instance.is_disabled:
        return self.instance.field_2
    else:
        return self.cleaned_data.get('field_2')