If I book a mixed cabin flight at the Business/First MilesAAver level (SLC to DFW in 1st, DFW to MIA in coach, MIA to LHR in business) and then later find out that 1st class opens up for DFW to MIA, do I have any options to get that? Or is it a done deal once I actually book it, with no options, even if first opens up? Do I still get to use the lounge and get priority boarding for the coach flight?