Personally I hate comment subjects because they populate the subject of notification emails, rather than saying "Reply to your comment" or "Reply to a comment" - that is, I can't tell just by looking at the subject line whether it's a direct reply to me or to someone else. But I agree that if you switch target reply-to comment without submitting, the subject chould be changed to the subject of the comment you've changed to, whether that's blank/default or some other user-entered text.