The id_value is not meant to be changed explicitly since it is a function of several company settings and previous values. The company settings that affect this are not fully implemented, but include clients_start, clients_increment, clients_pad_size, and clients_pad_str.
If you wanted to change the start value from 1500 to 2000, you could try the following (warning: I have not tested this):
<?php
...
$api = new BlestaApi($url, $user, $key);
$response = $api->put("companies", "setsetting", array('company_id' => 1, 'key' => "clients_start", 'value' => 2000));
...
?>