Changed the existing doc_to_period_id relationship into a one-to-many field. These were the steps:

• Create a docs_to_docPeriods table:

  --
  -- Table structure for table `docs_to_docPeriods`
  --
  
  CREATE TABLE IF NOT EXISTS `docs_to_docPeriods` (
    `dtp_dtp_id` int(11) NOT NULL auto_increment,
    `dtp_doc_id_fk` int(11) NOT NULL,
    `dtp_docPeriod_id_fk` int(11) NOT NULL,
    PRIMARY KEY  (`dtp_dtp_id`),
    KEY `dtp_docPeriod_id_fk_idx` (`dtp_docPeriod_id_fk`),
    KEY `dtp_doc_id_fk_idx` (`dtp_doc_id_fk`)
  ) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=1 ;
  

• Add constraints to it:

  --
  -- Constraints for table `docs_to_docPeriods`
  --
  ALTER TABLE `docs_to_docPeriods`
    ADD CONSTRAINT `docs_to_docPeriods_ibfk_1` FOREIGN KEY (`dtp_doc_id_fk`) REFERENCES `documents` (`doc_id`) ON DELETE CASCADE ON UPDATE CASCADE,
    ADD CONSTRAINT `docs_to_docPeriods_ibfk_2` FOREIGN KEY (`dtp_docPeriod_id_fk`) REFERENCES `docPeriods` (`dp_id`) ON DELETE CASCADE ON UPDATE CASCADE;
  
  

• Copy the existing data over to it:

  INSERT INTO `docs_to_docPeriods` (`dtp_doc_id_fk`, `dtp_docPeriod_id_fk`) SELECT `doc_id`, `doc_to_docPeriods_id` FROM `documents`
  

• Add the support for this in local_classes.php:

  $this->addField(new MdhOneToManyField('doc_periods', 'Period(s)',
                                  'doc_id', 
                                  'disPeriods',
                                  'dp_id',
                                  'dp_name',
                                  'docs_to_docPeriods',
                                  'dtp_dtp_id',
                                  'dtp_doc_id_fk',
                                  'dtp_docPeriod_id_fk',
                                  true, 'dp_name', true));
        

• Comment out the original field (we'll leave the data in there for a while, just in case).
• Test on dev db, then migrate to live.