нещо друго си сбъркал :

mysql> select ID from files;

+----+

| ID |

+----+

| 10 |

| 37 |

| 82 |

| 85 |

+----+

4 rows in set (0.00 sec)

mysql> update files set ID=ID-4;

Query OK, 4 rows affected (0.00 sec)

Rows matched: 4  Changed: 4  Warnings: 0

mysql> select ID from files;

+----+

| ID |

+----+

|  6 |

| 33 |

| 78 |

| 81 |

+----+

4 rows in set (0.00 sec)