One other note, Hertz allows you to modify a reservation, so that in most cases you can just change the rate on a current reservation without having to make a new one and cancel an old one.

If you can't do it online (which has never been an issue, but in a recent thread - http://www.flyertalk.com/forum/hertz...-sold-out.html - it was mentioned as a problem) you can call and they will give you the lower rate.

But you need to change the rate, they will not do it for you no matter how low the rate goes. You will keep the rate you reserved unless you make a change to it.