Wirelessly posted (HTC-8500/1.2 Mozilla/4.0 (compatible; MSIE 6.0; Windows CE; IEMobile 7.6))
Except for 'cool cars', I don't think there are really any upgrades from a luxury booking (unless you consider an SUV an upgrade). For a Luxury, you can expect a Caddi DTS or Lincoln Towncar. I don't know any US sites to carry Audis or Jags, unfortunately.
Even if they had any upgrades, if you're not a preferred/first/pres club/chairman member, an upgrade would be hard to come by.