[Andreas]

PHP Code:

<?php

error_reporting(E_ALL & ~E_NOTICE);

if (!is_object($DB_site))
{
    exit;
}

$DB_site->query("UPDATE " . TABLE_PREFIX . "thread SET forumid = B WHERE forumid = A");

require_once('./includes/functions_databuild.php');
build_forum_counters(A);
build_forum_counters(B);

log_cron_action('Threads moved from Forum A to Forum B', $nextitem); 

?>

Please note that this (very easy) Script will not bother updating users postcounts/titles.