I'd have to disagree Tony. If you want a bigger car, then don't wait to get to Orlando before upgrading.
When we've had I4s in the past, they will offer this and that at $xx/day to upgrade, but when we've declined, they've always backed off, and we've been left with the I4 we'd paid for.
Perhaps we've been lucky (or unlucky) with the rental agents we've had! We did get a free upgrade once (compact to I4), but they seem to have plenty of I4s available! Curiously they don't appear to have large numbers of premiums cars, we've been upgraded to minivan each time we've booked that size!
Ian