code: def raise_on_changed_subject_identifier(self):
          
          if self.id and self.subject_identifier_is_set():
              with transaction.atomic():
                  obj = self.__class__.objects.get(pk=self.id)
                  if obj.subject_identifier != self.subject_identifier_as_pk.hex:
                      if self.subject_identifier != obj.subject_identifier:
                          raise RegisteredSubjectError(
                              'Subject identifier cannot be changed for '
                              'existing registered subject. Got {} <> {}.'.format(
                                  self.subject_identifier, obj.subject_identifier))
docs: Raises an exception if there is an attempt to change
          the subject identifier for an existing instance if the subject
          identifier is already set.