I'm a President's Club member and always get upgraded at MCO. We usually reserve a mid or full size, and they usually have a Chrysler 300 (premium) waiting for us. I've successfully swapped that out for a convertible a couple of times (Mustang once and Sebring once) just because my husband wanted a convertible.
They seem to be pretty good there as we usually get a low mileage car that is clean and in good shape, not smoked in, etc. We've also never had to wait when turning the car in -- someone approaches us immediately and checks us out with the handheld invoice printer, and we walk a few steps into the terminal. YMMV. Hope you have a good trip.
Susan