テーブルの文字コード変換でデータ型が変わってしまう

foo.sql

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.6.27    |
+-----------+
1 row in set (0.00 sec)

mysql> create table a(
    -> a text
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> show create table a\G
*************************** 1. row ***************************
       Table: a
Create Table: CREATE TABLE `a` (
  `a` text
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> desc a;
+-------+------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------+------+-----+---------+-------+
| a     | text | YES  |     | NULL    |       |
+-------+------+------+-----+---------+-------+
1 row in set (0.01 sec)

mysql>  alter table a convert to character set utf8mb4;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show create table a\G
*************************** 1. row ***************************
       Table: a
Create Table: CREATE TABLE `a` (
  `a` mediumtext
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)

mysql> desc a;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| a     | mediumtext | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
1 row in set (0.01 sec)