[Andreas]

That is easy

PHP Code:

<?php
error_reporting(E_ALL & ~E_NOTICE);

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

$DB_site->query("UPDATE " . TABLE_PREFIX . "user SET usergroupid = 2 WHERE usergroupid = 677 AND lastactivity < " . (TIMENOW-X*86400));

log_cron_action('Inactive VIP Users reset to Registered', $nextitem);
?>

Replace X with the number of days, save this a a php-file in includes/cron, create a cron-job for it (to be run once/day for example) and you're done.