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