Excuse me if I'm repeating common knowledge -- someone may have binova this already... (...sorry, I didn't get the full "pova" in there...just the "ova". If I come up with something better, I'll come back and edit the post.)
To edit the subject: the original poster should click the little "edit" link at the bottom right of the original post. Then click the "Use Full Editor" button at the bottom of the edit area. The subject line will then appear at the top of the edit area, and the text can be modified or completely changed.
Cheers,
Jim