I don't understand the issue. That is technically an upgrade. You may want to check your contract and make sure they didn't change the rate on you...
If you don't like the assigned vehicle, you should tell them before you leave the counter with the paperwork. To make it easiest, what you'd want to do is ask them as they are assigning you the vehicle what vehicle you are getting. I learned to do this after Avis gave me pick up trucks twice and I didn't find out until I got out to the lot.