I have had several situations in which I was "upgraded" without charge to a car in a higher class. I have had so many bad experiences with GM rentals I try to avoid them as much as possible (fewest bad experiences have been with Pontiacs and Buicks, so if only GMs are available, I go for those). I have "downgraded" my rental on several occasions to get better quality cars in lower classes, such as Camry's or Corolla's. I never asked for or was offered a reduced rate when I do this, since the few dollars I might have saved in these situations were not worth the add'l time.

By the way, I have on several times been stuck with a vehicle I did not want (too large, not enough trunk space, etc.) because the airport location was out of other sizes. With at least most of the majors, you can go (and I have gone) to another of their rental locations and do a vehicle exchange. The neighborhood location might be glad to get that barge while you take one of their 10 mid-sizes. I've only run into one situation in which the location was not allowed to do this because of some franchise requirement, although I felt like the manager would have if he could.