I always advocate arming yourself with indisputable information, however all they'll probably start doing is claiming the upgrade "isn't available". Short of jumping onto their computer and accessing their fleet utilization records for the duration of your stay, there isn't much you can do, even if you booked a 'C' and spot a dozen Impalas on the ready line.

And to the OP, I'm sorry to hear about your terrible experience. Involuntary downgrades are my number one pet peeve in car rental. Either give me what I booked, or give me something a class up. That's the way it's always been, and the way it always should be.