When someone replies to an email from you, the subject has "Re:" inserted, doesn't it? So, why do you want to change the subject? It should remain the same. The "Re:" clearly indicates it's a reply, not an original message. And, if you reply back to such a message, the subject line will start with "Re: re:". That creates a sort of a trail which can be useful in recognizing the history of the message. So, why try to change the subject line?