I agree that the way a typical airport car rental works is that you pay the daily rate for the first 5 days (with a day defined as a 24-hour period). At that point, you're on the weekly rate for 5, 6, or 7 days, which is equivalent to 5 daily rentals. After that, the daily rate kicks in for the next 5 days days above the weekly rate, and so on.
There are some exceptions for promotional and weekend rates. For example, if your promotional rate requires keeping your rental car over a Saturday night, but you return the car before you've done so, you may be subject to a higher rate. Or if your weekend rental is only valid from noon Thursday to noon Monday, but you keep your car beyond that time window, you may be charged a higher rate. With reservations booked through Priceline or other discount brokers, early or late returns can be quite expensive.
In the OP's case, the two choices are to modify the reservation or simply to return the car early. If the original rate is no longer offered, modifying the reservation may result in higher total, even if the rental period is shorter. With the contract rate I usually use, returning the car early is not a problem, but it could be with some rates.
I would recommend to the OP simply call the rental car company's 800 number to ask the question, "What would I be charged if I return the car the day before it's due back?"