I did this once by accident at IAD. Reserved an XJ8, then decided to cancel and booked a midsize instead. Showed up and noticed that both reservations were still on the board -- I jokingly mentioned to the Gold agent that I would have to take the midsize, unless they could give me a deal on the XJ8 upgrade. She ended up giving me the XJ8 for my midsize rate plus a $25/day upgrade fee. ^
I thought about doing something similar again, but I heard that some airport locations will charge a no-show fee for Prestige vehicles. Never really found out if this is true though...