Write your forwards methods here.

def forwards(self, orm):
        "Write your forwards methods here."
        fields = orm['avocado.DataField'].objects.filter(app_name='variants',
            model_name='evs', field_name__in=('all_maf', 'aa_maf', 'ea_maf'))
        for f in fields:
            f.field_name = f.field_name.replace('maf', 'af')
            f.save()