Twitter was useless. After multiple written complaints to Hertz Executive Customer Relations, I eventually obtained a refund consistent with repricing my prepaid rental to a shorter duration -- as the T&C's say is permitted, though Hertz protested a lot, and staff are not trained accordingly. So, I got the outcome I sought.

Your situation seems different to me: You seek to not reprice the res: You just want to return it early and not be charged the higher rate. I know a bit about this situation from non-prepaid rentals, but I do not feel I know how this is handled on prepaid rentals.