I don't think there's a single travel business, be it hotel, airlines, cruise line, car rental, etc that will lower the rate and just give it to you without asking. If a hotel or airfare goes down, you just don't get the lower rate without changing your own reservation. It's the same thing with car rentals, you agree to a price, and if the price goes down it's up to you to see the rate dropped, and change your reservation.

Just like if you had booked a hotel room, an airline ticket, or booked a car with any other rental company.

Second, there is no need to cancel and rebook Hertz reservations. You open the reservation, click view modify, and if the rate is lower, just accept the lower rate. The reservation will change to the lower rate.