code: public static JobID downgrade(org.apache.hadoop.mapreduce.JobID old) {
      if (old instanceof JobID) {
        return (JobID) old;
      } else {
        return new JobID(old.getJtIdentifier(), old.getId());
      }
    }
docs: Downgrade a new JobID to an old one
  @param old a new or old JobID
  @return either old or a new JobID build to match old