When someone(usually a woman) trades his/her current situation, spouse and overall lifestyle for a better, more extravangant one. It is similar to gold digger, but normally the ones who are "Trading Up" can live on their own and already have money, but want more along with the fairy-tale love aspect of life.
The model from Candace Bushnell's book, "Trading Up" traded up her life as a model whore, to be the wife of a Film Company executive, but in turn traded up her life as a wife, to try and become a movie executive, herself.
In a sports stadium, 'trading' your crappy box seats for something a little closer to the action. Meaning, if there's nobody to trade the seat, why not just take it yourself?
Generally easy to do if a crappy team plays there because of the number of empty seats. It gets more difficult the better your team is, since the stadium will be close to sold out.
Sometimes involves dodging the wrath of particularly stern ushers. Also, if someone shows up with a ticket for your new seat, it's best to move.
There was nobody at list night's Tigers game, so I decided to do a little trading up. I left my crappy nosebleed section seat for a spot up close to the action.