mysql修改表的编码后怎么恢复已有的数据？

Question

举个例子，我创建表时没有设定字符集，默认是latin1，后来发现中文全乱码，就改成gbk。新插入的数据就正常了，但是已有的数据还是乱码，怎么让已有数据也恢复正常。下面是我的一个例子：

mysql> show create table example2 \G;
*************************** 1. row ***************************
Table: example2
Create Table: CREATE TABLE example2 (
Id int(11) NOT NULL AUTO_INCREMENT,
a varchar(20) DEFAULT NULL,
b date DEFAULT NULL,
PRIMARY KEY (Id)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

ERROR:
No query specified

mysql> ALTER TABLE example2 CONVERT TO CHARACTER SET gbk;
Query OK, 1 row affected (1.49 sec)
Records: 1 Duplicates: 0 Warnings: 0

mysql> select * from example2;
+----+------+------------+
| Id | a | b |
+----+------+------------+
| 1 | ?? | 2014-07-24 |
+----+------+------------+
1 row in set (0.00 sec)

mysql> insert example2 values(2,'我',now());
Query OK, 1 row affected, 1 warning (0.08 sec)

mysql> select * from example2;
+----+------+------------+
| Id | a | b |
+----+------+------------+
| 1 | ?? | 2014-07-24 |
| 2 | 我 | 2014-07-24 |
+----+------+------------+
2 rows in set (0.00 sec)

mysql>