In my experience, car rental companies do not guarantee a particular make and model, only a car category - e.g., economy, compact, midsize, full size. According to Avis, a Ford Focus is a compact car, as is a VW Beetle. They substituted one compact model for another compact model. They categorize a Toyota Corolla as a midsize car. Therefore, that would be an upgrade. It doesn't sound like bait and switch to me, even though it would have been an inconvenience for you.

Usually, there isn't too much difference in cost - how much extra did you have to pay?