J,
I want to remove an erroneous hashtag from someone's thread but don't
want to split the thread, which would result if I change the actual
title.
If you're talking about using "Edit Topic" item from the More menu on a message in the archive, then I think you can edit the Subject freely and it will affect the whole topic (thread) without splitting it.
Now if you're asking about the effect that has on subsequent replies to messages already posted, I'm not sure. I'm sure Mark said something in beta@ about having to keep the old subject(s) so that future replies would still match the thread, but I don't recall if he did it. Given that he already keeps old copies of message bodies when they're edited it doesn't seem too big a stretch to do the same with Subject lines.
But even if not, I thought at one point it was known that hashtags were excluded from the Subject match for threading purposes. But it has been a long time.
Either way this is something that you could learn in a test group before making the change in a "live" group.
Shal
Shal Farley