Car rentals usually run on a 24 hour clock. If the time the vehicle is in the renter's possession exceeds 24 hours, they should expect the bill to increase. The increase can vary based on the rental contract, with extra charges by the hour and scaling to the day a usual set of conditions.
If you change your pickup time to 8 hours earlier AND change your dropoff time to be eight hours earlier, your rate may remain the same, assuming that your original rate wasn't conditional on picking a car up after noon, which can happpen on "weekend rates."