[Xenon]

well, you have to change the userid od the first post of those threads, and then run the update thread function

so like that:

PHP Code:

$posts = $DB_site->query("
    SELECT post.postid, thread.threadid
    FROM thread
    INNER JOIN post ON (post.postid = thread.firstpostid)
    WHERE thread.forumid = ZZ
        AND post.userid = AAA
    ORDER BY RAND()
    LIMIT XX
");
require_once('./includes/functions_databuild.php');

while ($post = $DB_site->fetch_array($posts))
{
    $DB_site->query("UPDATE post SET userid = BBB WHERE postid = $post[postid]");
    build_thread_counters($post['threadid']);
} 


note, that it's not the optimal way to do it, as it produces x queries per post, but if your limit isn't too big, it should work