I am sure you are talking about the old Infopop script that was used for the move from 5.x to 6.
The problem with a move to threads is that a thread number is defined as the overall thread number and not just the thread number in a specific forum.
In threads all posts are in the same table whereas in classic the next thread number deponds only the threads in that single forum.
Thus to do a link redirect you need to add (in a database for example) a reference of the old forum/thread number and the new threads thread number as there is no way to know it so it really has to be done during the import.
