Check out the following page: https://www.southwest.com/html/air/samedaychange.html

Of particular note to your last question, from the above linked page: *A-List and A-List Preferred Members can take advantage of free same-day change and same-day standby benefits, even with Wanna Get Away fares. So if you are A-List or A-List Preferred, you won't have to pay a fare difference even if you have a base WGA fare. If you aren't an elite, then you would pay the fare difference if on a WGA fare.