very nice idea Xenon.
May I suggest an improvement? Maybe you can add another query to $action=="newreply" section to check if he is the last poster to the thread. If he is, he can be pre-warned in the screen he writes his message that his last message will be replaced with this one.
This will add an extra query to newreply.php but it can save the user from losing his last message in the thread especially when the hack is new in the board.