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