code: def update_thesis_information(self):
          
          fields_501 = record_get_field_instances(self.record, '502')
          for field in fields_501:
              new_subs = []
              for key, value in field[0]:
                  if key == 'b':
                      new_subs.append(('a', value))
                  elif key == 'c':
                      new_subs.append(('b', value))
                  elif key == 'd':
                      new_subs.append(('c', value))
                  else:
                      new_subs.append((key, value))
              record_delete_field(self.record, tag="502",
                                  field_position_global=field[4])
              record_add_field(self.record, "502", subfields=new_subs)
docs: 501 degree info - move subfields.