Mmm, lets see
viewtopic.php
FIND
result = $db->query('SELECT id FROM '.$db->prefix.'posts WHERE topic_id='.$id.' ORDER BY posted') or error('Unable to fetch post info', __FILE__, __LINE__, $db->error());
REPLACE WITH
result = $db->query('SELECT id FROM '.$db->prefix.'posts WHERE topic_id='.$id.' ORDER BY posted DESC') or error('Unable to fetch post info', __FILE__, __LINE__, $db->error());
FIND
$result = $db->query('SELECT u.email, u.title, u.url, u.location, u.use_avatar, u.signature, u.email_setting, u.num_posts, u.registered, u.admin_note, p.id, p.poster AS username, p.poster_id, p.poster_ip, p.poster_email, p.message, p.hide_smilies, p.posted, p.edited, p.edited_by, g.g_id, g.g_user_title, o.user_id AS is_online FROM '.$db->prefix.'posts AS p INNER JOIN '.$db->prefix.'users AS u ON u.id=p.poster_id INNER JOIN '.$db->prefix.'groups AS g ON g.g_id=u.group_id LEFT JOIN '.$db->prefix.'online AS o ON (o.user_id=u.id AND o.user_id!=1 AND o.idle=0) WHERE p.topic_id='.$id.' ORDER BY p.id LIMIT '.$start_from.','.$pun_user['disp_posts'], true) or error('Unable to fetch post info', __FILE__, __LINE__, $db->error());
REPLACE WITH
$result = $db->query('SELECT u.email, u.title, u.url, u.location, u.use_avatar, u.signature, u.email_setting, u.num_posts, u.registered, u.admin_note, p.id, p.poster AS username, p.poster_id, p.poster_ip, p.poster_email, p.message, p.hide_smilies, p.posted, p.edited, p.edited_by, g.g_id, g.g_user_title, o.user_id AS is_online FROM '.$db->prefix.'posts AS p INNER JOIN '.$db->prefix.'users AS u ON u.id=p.poster_id INNER JOIN '.$db->prefix.'groups AS g ON g.g_id=u.group_id LEFT JOIN '.$db->prefix.'online AS o ON (o.user_id=u.id AND o.user_id!=1 AND o.idle=0) WHERE p.topic_id='.$id.' ORDER BY p.id DESC LIMIT '.$start_from.','.$pun_user['disp_posts'], true) or error('Unable to fetch post info', __FILE__, __LINE__, $db->error());
I think that should properly reverse the view order of posts.
Out of curiosity, why would you want this? Reversing the order of posts makes the conversation much more difficult for people to read if they haven't kept up with the topic and doesn't provide much benefit (if any) for people who have 
