> Why won't just changing the subject line start a new thread?

Because user mail agents thread on the Message-id:, References:, and 
In-reply-to: fields in the headers of the messages, but NOT the Subject: 
line.

For the whole huge, gory, story read the Request For Comments paper by the 
Internet Engineering Task Force.

http://www.faqs.org/rfcs/rfc2822.html

Section: 3.6.4. Identification fields

I wish more people knew this because the frequency of thread hi-jacks by 
otherwise well-mannered people is getting out of hand again. It would seem to 
be a bit of Net-Lore which needs re-iterating from time to time.

--
CS

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