If you want and need an SUV you should reserve an SUV in the first place. With a fullsize reservation, even with PC status, it's far from guaranteed that they will be willing to give you an SUV. If they want to be nice and they have one available, then maybe, but if for example, they have have confirmed reservations for others for SUVs, they're not going to be able to accommodate you even if they wanted to.

Airport locations obviously have more stock on-hand, but there is no guarantee that they'd let you swap to an entirely different class, so I definitely would not count on it. It would be at the discretion of the on-duty manager and your powers of persuasion may come into play as well.