There's just one line to change if you want to do it by yourself:
includes/entry.inc.php, line 92:

replace

if(isset($child_array)) $tree[] = tree($entrydata['tid'], $child_array);

by

if(isset($child_array)) $tree[] = tree($entrydata['id'], $child_array);

this didn't work exactly, it cuts off replies made to posts earlier in the thread. I want the structure to remain the same, just not to show higher up posts.

This forum software does it like this: http://b4.boards2go.com/boards/board.cgi?&user=tamalpa

Is a quick fix possible?