MySQLWorkbenchが表示したSQLが下記
ALTER TABLE `demo`.`group_users`
DROP FOREIGN KEY `fk_group_users_division_id_divisions_id`;
ALTER TABLE `demo`.`group_users`
CHANGE COLUMN `division_id` `division_id` INT(11) UNSIGNED NULL DEFAULT NULL COMMENT '[区分id]' ;
ALTER TABLE `demo`.`group_users`
ADD CONSTRAINT `fk_group_users_division_id_divisions_id`
FOREIGN KEY (`division_id`)
REFERENCES `demo`.`divisions` (`id`);