I have a pre-paid reservation and tried to modify the pick up time from 8:30AM to 7:30 AM- no change to days of week or length of rental.
The online system would not allow any changes to be made to the reservation and advised me to call customer service.
The Hertz service rep said that their system does not allow any changes to prepaid reservations but if the car is available, the rental office will release early.

Is it true that no modification of pick up time for prepaid rentals are allowed at all? What happens if an air line schedule is delayed or changed?
I am President's Circle but that doesn't seem to make any difference.