Imagine everyone discussing about something read later the thread's been changed into "Justin Bieber fans, let's discuss here!"
I wrote here (along the thread), that what can be done is to have a two links (one with the older name and one with the new name) that will point to the same thread.
Moreover, it could be defined that who will read a thread will always see only the proper link (thread's name).
In addition, the current name could be added to the header, below the "link" name.