Luke R,
Passengers don't actually pay for their seating position. They pay for their class of travel ie. first, business or economy. If your scenario was to occur (assuming a two class cabin) passengers seated in the first row(s) of economy would be asked to move aft. If the problem still existed passengers in business might be asked to move to the back of their class, or alternatively (as Checkboard discovered in reverse) into economy. They would sit there for take-off and landing but spend the majority of the flight in their ticketed class.
J_T,
I've always been curious as to how Southwest structured their loading system but have never been able to find anyone who could cast light on the subject. The stabtrim issue is an interesting one - I'm not sure how they've solved it. Maybe another posted could enlighten us.