If you get to the airport on the day of the flight and seats are still available for purchase on the flight that you want to travel on(go on the website and put in the max number of people possible, don't bother with the seat map), you can be confirmed as long as your original flight was on the same day for $50.

You can also go standby, in which case there is no cost but don't bother trying to find out if you're going to get on until 45 minutes prior to departure at the gate at the earliest.

No they will not upgrade people if your cabin is full in order to put you on.
Hope that helps.