外部キー制約が存在する場合のMySQL主キーの変更

主キーを削除する前に、old_pkにインデックス（UNIQUEの場合もあります）を追加します：

mysql> CREATE TABLE parent (
    ->     old_pk CHAR(8) NOT NULL PRIMARY KEY
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE child (
    ->     parent_key CHAR(8),
    ->     FOREIGN KEY (parent_key) REFERENCES parent(old_pk)
    ->         ON UPDATE CASCADE ON DELETE CASCADE
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO parent VALUES ('a');
Query OK, 1 row affected (0.01 sec)

mysql> CREATE INDEX old_pk_unique ON parent (old_pk);
Query OK, 1 row affected (0.01 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE parent DROP PRIMARY KEY;
Query OK, 1 row affected (0.01 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> INSERT INTO child VALUES ('a');
Query OK, 1 row affected (0.00 sec)

mysql> SHOW CREATE TABLE parent;
+--------+------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                 |
+--------+------------------------------------------------------------------------------------------------------------------------------+
| parent | CREATE TABLE `parent` (
  `old_pk` char(8) NOT NULL,
  KEY `old_pk_unique` (`old_pk`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+--------+------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> INSERT INTO child VALUES ('b');
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test/child`, CONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_key`) REFERENCES `parent` (`old_pk`) ON DELETE CASCADE ON UPDATE CASCADE)

mysql> INSERT INTO parent VALUES ('b');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO child VALUES ('b');
Query OK, 1 row affected (0.01 sec)

mysql> ALTER TABLE parent ADD id INT;
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> UPDATE parent SET id = 1 WHERE old_pk = 'a';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> UPDATE parent SET id = 2 WHERE old_pk = 'b';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> ALTER TABLE parent ADD PRIMARY KEY (id);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE parent;
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                             |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| parent | CREATE TABLE `parent` (
  `old_pk` char(8) NOT NULL,
  `id` int(11) NOT NULL default '0',
  PRIMARY KEY  (`id`),
  KEY `old_pk_unique` (`old_pk`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)