Have you prepaid the reservation already? That is the only scenario I am aware of where you cannot modify the reservation.

I would expect the Corvette C6s to be gone, but they do tend to stay on the fleet for a long time, so I wouldn't take it for granted.

I would make a flexible reservation for a standard convertible - you're almost guaranteed a Mustang or Camaro V6 and frankly, the Camaro is by far preferable to the pre-2015 Mustang anyway.

Edit: add PC 192021 to your reservation with whatever discount code you're already using and you should get a flexible rate for a similar or lower price than the prepaid option.