Possible to manually change links for threads?

[Dave]

If you only want to do it against a few threads, you can just write a few .htaccess rewrite rules (without redirect) that when a specific URL is called, they internally call a different URL.

However if it has to be more flexible, you're going to have to create a dynamic .htaccess rewrite rule and additionally modify some files (mainly showthread.php) which will take too much time/work for me to explain to you in a post here.