(1.5) What do I do when the thread has evolved into a topic that is no longer appropriate for the newsgroup?

When that happens, you should move the discussion to the most appropriate
newsgroup and maybe even change the subject line as well. There are several
ways to execute this transition :

1) Do one last response of this thread in the original newsgroup.
   Somewhere in the post, mention that you think the discussion should
   be moved to a new newsgroup.
   Set the follow-up of the post to this new newsgroup.
   (i.e. in header, put, Followup-To: new_newsgroup)

2) If only a few people are involved in this particular thread, email to
   inform the several individuals about the change of the forum for future
   discussion.

Better yet, do both (1) and (2).

, if the thread is "dying", don't bother to followup on it to
get it into another newsgroup. Your post will only keep the thread
alive a bit longer.