I'm no expert on US but I would bet on your ability to climb aboard an empty earlier flight. In other threads some posters have noted that gate agents do not always look for the obscure priceline rules if they are trying to fill a plane and make room on the next just in case.

You can register on the ITN website http://www.itn.net/cgi/get?itn/pl/amex_personal/index

(register as an expert in your profile)
and you can see the availability of individual fare bases on individual flights). You can also view seatmaps. Knowing loading will tell you your likelihood of getting on an earlier flight and which one.

Course you could call the res desk for a synopsis of the info. But that takes the fun out huh. Good luck!