mysql> create table foo (id int(10) primary key auto_increment);
Query OK, 0 rows affected (0.22 sec)

mysql> alter table foo add column bar varchar(12);
Query OK, 0 rows affected (0.37 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> insert into foo set bar="BLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAHBLAH";
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> select * from foo;
+----+--------------+
| id | bar          |
+----+--------------+
|  1 | BLAHBLAHBLAH |
+----+--------------+
1 row in set (0.00 sec)

mysql> alter table foo modify bar varchar(256);
Query OK, 1 row affected (0.05 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from foo;
+----+--------------+
| id | bar          |
+----+--------------+
|  1 | BLAHBLAHBLAH |
+----+--------------+
1 row in set (0.00 sec)

mysql>