Quoting g4sra via Dng (dng@???):

> What is the netiquette on this list for changing subject lines.
> I hate hijacking threads but I also hate breaking them.
> Which is the lesser of the two evils ?

Standards-compliant mailers' threading doesn't get broken by altering
the Subject header. (Last I heard, MS-Outlook still didn't follow
the RFCs in that area, and thus gets adversely affected. Probably,
some other common mailers share that defect.)

In a standards-compliant mailer that relies on Message-IDs and the
In-Reply-To header to arrange threading, you will continue to see a
reply with changed Subject header sorted immediately below the
replied-to post within the conversation's tree of postings.

Therefore, changing Subject lines and breaking or not breaking threads
are (or, at least, should be) orthogonal concerns. If you wish to
deliberately break threading in your reply, snip out the In-Reply-To
from your headers before posting. If you don't wish to, then don't
do that. Either way, changing or not changing the Subject header ought
to have zero effect on threading (except for MS-Outlook victims^W
users).

Since you're a GMail person, you _may_ find it to be non-trivial to assert
control over your SMTP headers, but I wish you good hunting, on that.
I'll be polite and not say how I prefer to cure GMail problems. (It's
very similar to how I cure Facebook problems.)

Personally, I deliberately break the thread by snipping In-Reply-To (or
use mutt's new-message command, which amounts to the same thing) if the
new discussion will be semantically quite different from the old one.
OTOH, if I'm just adjusting the Subject header to account for topic
drift, I don't break threading because I regard it as part of the
ongoing conversation.