[MarkFL]

Create a plugin hooked at "threaddata_presave" with the code:

PHP Code:

global $vbulletin;

if ($vbulletin->GPC['threadid'] == XXX)
{
    $vbulletin->db->query_write("
        UPDATE " . TABLE_PREFIX . "post
        SET title = '" . $vbulletin->db->escape_string($vbulletin->GPC['title']) . "'
        WHERE threadid = " . $vbulletin->GPC['threadid'] . "
        AND parentid != 0
    ");
} 


Replace "XXX" with the threadid of the thread you want affected.