Trying to use some Emerald Club Free Days in ATL.

It appears I can't specifically reserve a "Luxury" vehicle with free days.

I made my reservation for a "Premium SUV" which was listed at $125.99/day and applied some of my free days.

I then signed out and built a dummy reservation (same dates/times) and the system showed "Luxury" class vehicles available for $79.99/day.

Is there any way to better my chances for a luxury vehicle? I'm sure if I walk up with my reservation and ask to "upgrade" to a luxury class car, they'll charge me the difference at that time. However, at the time of booking, the difference was negative!