tukar nilai field

DROP TABLE IF EXISTS tabel;
CREATE TABLE IF NOT EXISTS tabel(
id INT AUTO_INCREMENT PRIMARY KEY,
absen VARCHAR(2)
);

INSERT INTO tabel(absen)
VALUES
('A'),
('S'),
('I'),
('I'),
('A'),
('S'),
('S'),
('I'),
('A');

SELECT * FROM tabel;
+----+-------+
| id | absen |
+----+-------+
|  1 | A     |
|  2 | S     |
|  3 | I     |
|  4 | I     |
|  5 | A     |
|  6 | S     |
|  7 | S     |
|  8 | I     |
|  9 | A     |
+----+-------+
9 rows in set (0.00 sec)

UPDATE
tabel
SET absen=IF(absen='A','I',IF(absen='I','A',absen));

SELECT * FROM tabel;
+----+-------+
| id | absen |
+----+-------+
|  1 | I     |
|  2 | S     |
|  3 | A     |
|  4 | A     |
|  5 | I     |
|  6 | S     |
|  7 | S     |
|  8 | A     |
|  9 | I     |
+----+-------+
9 rows in set (0.00 sec)