mysql> select * from swaptable;
+----+------+
| id | num  |
+----+------+
|  5 |  100 |
|  6 |  200 |
|  7 |  300 |
+----+------+
3 rows in set (0.00 sec)

mysql> UPDATE swaptable SET id = CASE id WHEN 5 THEN 6 
    -> WHEN 6 THEN 5 END 
    -> WHERE id IN (5,6);
Query OK, 2 rows affected (0.01 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from swaptable;
+----+------+
| id | num  |
+----+------+
|  6 |  100 |
|  5 |  200 |
|  7 |  300 |
+----+------+
3 rows in set (0.00 sec)

mysql> UPDATE swaptable SET id = CASE id WHEN 5 THEN 6 
    -> WHEN 6 THEN 5 END 
    -> WHERE id IN (5,6);
Query OK, 2 rows affected (0.01 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from swaptable;
+----+------+
| id | num  |
+----+------+
|  5 |  100 |
|  6 |  200 |
|  7 |  300 |
+----+------+
3 rows in set (0.00 sec)