正确的改变table字符集的语句是：
alter table xxx convert to character set utf8;
而不是想当然的:
alter table xxx default charset utf8;

字符集从GBK转成utf8，
会增大字段所占用的空间，有可能会改变字段的类型:
比如text有可能会自动变成medium text
但是varchar没有自动变成medium text.

下面看测试：

建一张GBK的表：
mysql> SHOW CREATE TABLE xxx;
CREATE TABLE `xxx` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`title` VARCHAR(255) NOT NULL DEFAULT '',
`body` text,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=gbk

用想当然的办法把它转成utf8:
mysql> ALTER TABLE xxx DEFAULT charset=utf8;
Query OK, 0 ROWS affected (0.13 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql>
mysql> SET names utf8;
Query OK, 0 ROWS affected (0.00 sec)

插入测试数据：
mysql> INSERT INTO xxx SET title='我爱北京天安门';
Query OK, 1 ROW affected (0.06 sec)

成功了。。。然后您就认为万事OK了？

mysql> INSERT INTO xxx SET title='㤇';
Query OK, 1 ROW affected, 1 warning (0.03 sec)
Warning (Code 1366): Incorrect string VALUE: '\xE3\xA4\x87' FOR COLUMN 'title' at ROW 1

注意 title和body字段的CHARACTER SET 仍然为gbk
mysql> SHOW CREATE TABLE xxx;
| xxx | CREATE TABLE `xxx` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`title` VARCHAR(255) CHARACTER SET gbk NOT NULL DEFAULT '',
`body` text CHARACTER SET gbk,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8 |

正确的做法:

mysql> DROP TABLE xxx;
Query OK, 0 ROWS affected (0.02 sec)

mysql> CREATE TABLE `xxx` (
-> `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
-> `title` VARCHAR(255) NOT NULL DEFAULT '',
-> `body` text,
-> PRIMARY KEY (`id`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=gbk;
Query OK, 0 ROWS affected (0.06 sec)

mysql> ALTER TABLE xxx CONVERT TO CHARACTER SET utf8;
Query OK, 0 ROWS affected (0.14 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> INSERT INTO xxx SET title='㤇';
Query OK, 1 ROW affected (0.04 sec)

mysql> SHOW CREATE TABLE xxx;
| xxx | CREATE TABLE `xxx` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`title` VARCHAR(255) NOT NULL DEFAULT '',
`body` mediumtext,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8

可是你注意到 body字段从text变成 mediumtext了吗？

text字段，最多存储65535字节，换成GBK的字符就是32767个字符，这32767个gbk字符转成utf8却要占用98301字节，已经超过text的存储能力，所以被自动转成了mediumtext。

下面测试VARCHAR的情况：

VARCHAR除了数据部分，还有1-2个字节用来保存数据的长度。如果只使用一个字节，那么长度上限为255(2^8-1)，如果使用二个字节，长度上限为65535(2^16-1)。
所以VARCHAR最多存储65535字节，换成GBK字符为32767个：
mysql> DROP TABLE xxx;
Query OK, 0 ROWS affected (0.02 sec)

mysql>
mysql> CREATE TABLE `xxx` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`title` VARCHAR(32768) NOT NULL DEFAULT '',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=gbk;
ERROR 1074 (42000): COLUMN LENGTH too big FOR COLUMN 'title' (MAX = 32767); USE BLOB OR TEXT instead
mysql>
mysql>
可是 32767也是不成的。。还有每行记录的总长度限制(不包括text和BLOB字段) 65535：
mysql> CREATE TABLE `xxx` ( `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, `title` VARCHAR(32767) NOT NULL DEFAULT '', PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=gbk;
ERROR 1118 (42000): ROW SIZE too LARGE. The maximum ROW SIZE FOR the used TABLE TYPE, NOT counting BLOBs, IS 65535. You have TO CHANGE SOME COLUMNS TO TEXT OR BLOBs
mysql> CREATE TABLE `xxx` ( `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, `title` VARCHAR(32766) NOT NULL DEFAULT '', PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=gbk;
ERROR 1118 (42000): ROW SIZE too LARGE. The maximum ROW SIZE FOR the used TABLE TYPE, NOT counting BLOBs, IS 65535. You have TO CHANGE SOME COLUMNS TO TEXT OR BLOBs
mysql> CREATE TABLE `xxx` ( `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, `title` VARCHAR(32765) NOT NULL DEFAULT '', PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=gbk;
ERROR 1118 (42000): ROW SIZE too LARGE. The maximum ROW SIZE FOR the used TABLE TYPE, NOT counting BLOBs, IS 65535. You have TO CHANGE SOME COLUMNS TO TEXT OR BLOBs
mysql> CREATE TABLE `xxx` ( `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, `title` VARCHAR(32764) NOT NULL DEFAULT '', PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=gbk;
Query OK, 0 ROWS affected (0.06 sec)

32764 * 2 + 2 + 4 = 65534 最接近于65535了，
32765 * 2 + 2 + 4 = 65536 超过65535。
*2是因为gbk字符占用2字节。
+2是VARCHAR还需要额外2字节保存数据的长度。
+4是id字段INT UNSIGNED占了4字节。
合理吗？

转换字符集为utf8之后，VARCHAR(32764) 已经不足以保存 32764个utf8字符：

mysql> ALTER TABLE xxx CONVERT TO CHARACTER SET utf8;
ERROR 1074 (42000): COLUMN LENGTH too big FOR COLUMN 'title' (MAX = 21845); USE BLOB OR TEXT instead

21845 * 3 = 65535.
 

仓促完成，难免有错，欢迎指出。