Free upgrade, never heard of it happening. Even when I was on first name basis with multiple folks we the Vette took a special release from the highest ranking manager in duty or something to the effect this was a year+ ago when Avis "exotics" we're fairly uncommon compared to Hertz. So IAH, you are the man!

Anyways as far as paying for an upgrade I can't think of the last time I've been to MCO and there wasn't a Vette sitting off to the side looking for someone to take it for a spin so I'd venture to say your chances there are pretty good. Remember MCO has two sides. So just because you don't see it when you get to the garage doesn't mean there isn't one available.