On instructions from JW, added a new table and two new fields to the db. Tested first in dev, then carried out the same changes in live. SQL:

CREATE TABLE IF NOT EXISTS `period` (
  `pd_id` INT NOT NULL AUTO_INCREMENT ,
  `pd_name`  varchar(128) collate utf8_unicode_ci default NULL ,
  PRIMARY KEY (`pd_id`) 
)
ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

INSERT INTO `period` (`pd_id`, `pd_name`) VALUES (1, '[Unassigned]');
INSERT INTO `period` (`pd_id`, `pd_name`) VALUES (2, 'AR');
INSERT INTO `period` (`pd_id`, `pd_name`) VALUES (3, 'FR');

ALTER TABLE `trial` ADD COLUMN `tr_period` INT(11) NULL AFTER `tr_cote`;
ALTER TABLE `trial` ADD COLUMN `tr_photos` INT(11) NULL AFTER `tr_description`;

ALTER TABLE `trial` 
ADD CONSTRAINT `trial_ibfk_3` FOREIGN KEY (`tr_period`) REFERENCES `period` (`pd_id`) ON DELETE SET NULL ON UPDATE CASCADE;

Corresponding changes made to local_classes.php. Also wrote a script to copy data from live to dev db, and fixed a minor bug in the dev_to_live_update.sh script.