It's OK to have a radical change in the message content in a thread,
as long as the replier is actually replying to _something_ in (or about)
that which they are replying to.
If the referenced email is what prompts the reply, that is sufficient, I would say.  The trail is supposed to demonstrate the chain of conversation, which will naturally change topic over time.
If Ian had started a new thread, it would have broken the reference
to what he was taking exception to. His continuation of the thread
was appropriate. My changing the subject was also appropriate.
I take issue with your last statement. In the previous 3 messages you'd sent, you had answered the same email:
'searching for kanji strings, ignore punctutation and end of lines. Text indexing and retrival in unicode.'
with:
'Re: Nasty problem: searching for strings that span newlines'
'Re: Regex efficiency'
'Re: Searching for kanji strings: Use UTF-8'
...and, notably, none with "Re: <original subject>", which would have been the
conventional means of answering the question, since you're not actually
changing the topic of conversation.
It means that, in order to be a good citizen, if I wish to reply
without just repeating you, I have to read 3 disparate emails to work
out what you've said before I reply.  (And, because my mailer
is not that good at threading, I can't even work out which ones they
are.)  
A more conventional mode of replying (on a technical list, at
least) would be to make one email with all the points you have to make
listed in a structured manner.  If someone then chooses to go off on a
tangent from 'searching for kanji strings...' then it would be
*their* responsibility to change the subject: e.g. "Spiny anteater
spotting in Greater Tokyo; was: ...".  If they pick up one point out of many, or an issue with the solution, they may, at their option, clarify so - "UTF8 regexps, was:...".
In fact, I struggle to find an email you've sent where you *haven't* changed the subject line.  I think there's a balance to be struck on this, and you're overshooting in the opposite direction from most people.
On a somewhat semantic point, you should probably remove the 'Re:'
if you change the subject, too, since an email entitled 'Re:
<stuff>' implies the existence of an email called '<stuff>'
to which you're replying, to my mind.
[1] 
http://www.tlug.jp/ML/0601/msg00144.html
[...]
In [1], Ian should have quoted _something_, particularly my subject,
and Ian should have changed the subject himself, but understandably
did not because he thought it wrong to do so (which was his point!).
I think you're right on both points, here, actually.  My bad.
I note that this is now way offtopic on the list's main subject, for which I apologise.
-- 
Ian.