Need trigger for guest/pet count change

Guests often enter an incorrect number of guests/pets when first booking, or need to make changes. When that number changes, I needs it to automatically trigger the rental agreement to be resent. There is not currently a way to trigger the rental agreement to be sent again automatically when the number of guests or pets changes, so I'm manually resending the rental agreement often, usually at least 2-3 times per week. That's if I remember and am in a position to resend it.