I have never been denied the car class I reserved and then offerred a higher class car for $$/day.

I have only experienced the opposite where they were out of the ultra-smal- super-tiny-teeny sub compact that my company forced us to rent and so they asked me if I would like the (larger fill in the blank here) instead at no extra charge. Two-three car classes upgrades are not unusual. At SJC where I use to rent 2-3x per week, I would rent the tiniest car available at Hertz and would always be angling for an upgrade to the Jaguar!