I'll add that I had a similar experience with Thrifty -- Memorial Day weekend at Love Field in Dallas. Advance reservations for a mid-sized sedan. Even an e-mail from Thrifty the day before pick-up to remind me of the reservation. But, when I arrived at the Thrifty location, I was told that they were 'out of mid-sized' sedans. The only thing that they 'had' available for my reservation was a minivan. They had upgrades available, though, for about double my reservation price.
I last made a reservation with Thrifty, way back in 2004, at SAT. Exact same experience. No sedans available, only a minivan. Upgrades were available, then, too.
Amazing how I can rent, over the three-year period, close to seventy-five cars from Hertz, Avis, and Alamo without problem. But, two visits to Thrifty and two bait-and-switch routines.