This probably isn?t the most efficient way to do it, but here is how I did my editpost.php file modification so that only when editing the first post would you be able to change the topic prefix. Changes are in the attached file. Use them in place of the modifications for editpost.php for this hack if you like my method of implementation.
All credit for this really does go to the original hack?s author. I just modified it for my needs and do thank him for the well-written and powerful hack.