full size to convertible is not a one class upgrade. That would be a Premium car and unless you are at a location with very low demand for convertible, I doubt they will hand you a convertible on a Premium reservation.

Unless you are willing to return to the rental location after the two days and exchange your convertible for a full size car, you would have to make either a convertible reservation for all 5 days or a full size reservation for all 5 days. Of course you have to pay the remaining 3 days based on the daily rate of your reservation. If you are making two reservations you would have to return to the rental location to close the first one and open the second one. Can't do over the phone and keep your car.