Upgrading from a full size to a premium (300, Maxima) technically does fulfill the rules of "Avis First" one class upgrades... it's the preferred booth's discretion if they upgrade you beyond 1class to your desired Luxury/Cool car choices.
When I was Avis first, I had much better success reserving a premium, getting luxury, or reserving luxury and getting a cool car (such as Mustang GT, Camaro SS, Challenger RT etc) for the most desired car models. I could often count on a multi-class upgrade when reserving a intermediate and getting a premium.
Unfortunately it sounds like the new Avis select and go has eroded the Avis elite upgrade success rates... luckily most of my rentals have been at non select&go locations!