Well, I got to watch someone at MCO have a screaming hissy fit at check in because they got charged for the mini van even though they took it from the standard car aisle. Part of their screaming fit was "we told the girl at the exit to cancel that reservation not the other one" And one of the things the agent said was "you checked in for this one" LOL!
But feel free to try it and if it doesn't work, that will give you something to post about LOL!
It's kind of like the NUMEROUS folks I saw over the years at the John Wayne Airport having hysterical fits because they would not fit in the car size they reserved but "assumed" they would be able to get mini van or SUV on the aisle for the same price. (That airport is space challanged, you get what you pay for there more often then not and the aisle upgrades are slim!)
I travel a lot and I see a lot of people trying "creative ways" to save money and a lot of resulting hysteria. (It doesn't offend me, I actually find it rather entertaining to hear people try to convince the airline, rental car company etc... that the screw up is not their fault. Last trip is was another family who just KNEW the airline would let them take the flight they really wanted for no additional charge instead of the one they didn't want that they booked because it was a lot cheaper. The Dad even patiently explained this to the agent as the reason he should get the free change to the "better flight". He went on to explain that flying home on the late night flight was just too rough on his kids so the airline shouldn't make them do it Didn't work! This was actually NOT Orlando, but....)