code: def _upgrade(self):
          
          logging.debug('[FeedbackResults]._upgrade()')
          if hasattr(self, 'version'):
              version = Version.fromstring(self.version)
          else:
              version = Version(0)
          logging.debug('[FeedbackResults] version=%s, class_version=%s' %
                        (str(version), self.class_version))
          if version > Version.fromstring(self.class_version):
              logging.debug('[FeedbackResults] version>class_version')
              raise FutureVersionError(Version.fromstring(self.class_version),
                                       version)
          elif version < Version.fromstring(self.class_version):
              if version < Version(0, 1):
                  self.calibration = FeedbackCalibration()
              if version < Version(0, 2):
                  # flag invalid data points
                  self.version = str(Version(0, 2))
                  self.fb_resistor[self.V_fb > 5] = -1
                  self.hv_resistor[self.V_hv > 5] = -1
              if version < Version(0, 3):
                  self.attempt = 0
              if version < Version(0, 4):
                  del self.sampling_time_ms
                  del self.delay_between_samples_ms
                  self.voltage = self.options.voltage
                  del self.options
                  del self.attempt
              if version < Version(0, 5):
                  self.area = 0
                  self.version = str(Version(0, 5))
              if version < Version(0, 6):
                  self.amplifier_gain = None
                  self.vgnd_hv = None
                  self.vgnd_fb = None
                  self.version = str(Version(0, 6))
                  logging.info('[FeedbackResults] upgrade to version %s' %
                               self.version)
          else:
              # Else the versions are equal and don't need to be upgraded.
              pass
docs: Upgrade the serialized object if necessary.
  
          Raises:
              FutureVersionError: file was written by a future version of the
                  software.