hide_user31@phone>create table test ( foo tinyint default null );
Query OK, 0 rows affected (0.01 sec)

hide_user31@phone>insert into test values (1),(0),(1);
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

hide_user31@phone>alter table test change column foo foo enum('zero','one');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

hide_user31@phone>select * from test;
+------+
| foo  |
+------+
| zero |
|      |
| zero |
+------+
3 rows in set (0.00 sec)