Yeah, that's a forum feature that can sometimes be quite inconvenient. Not something I can change myself and I'm not sure it's configurable (or if they would even want to do so, for that matter).
But if you want the subject to be changed I can do that, and add a note to the OP if you like. I wouldn't worry too much about the OP accurately predicting the evolving scope of the discussion, it's just what they do!
