code: private void setDeprecatedPropertiesForUpgrade(Properties props) {
      deprecatedProperties = new HashMap<String, String>();
      String md5 = props.getProperty(MESSAGE_DIGEST_PROPERTY);
      if (md5 != null) {
        deprecatedProperties.put(MESSAGE_DIGEST_PROPERTY, md5);
      }
    }
docs: Pull any properties out of the VERSION file that are from older
  versions of HDFS and only necessary during upgrade.