For what it's worth, I've gotten Mustang GT (U4) upgrades on a Standard convertible (U) two times in the last 6 months, for weeklong reservations @ OGG. They had a bunch of new Camaro SS cars on the lot, which I assume I could have swapped to if I wanted since they're same class, but I actually prefer the Mustang GT (smaller, easier to park, just as fast and way more usable trunk space).

Your chance of getting a corvette is approximately 0.0%, as an upgrade. You can probably pay to upgrade, but it's likely to be steep, on the order of ~$100+/day.