Have you ever been making your way through first class on your way to your seat in coach, spied an empty seat, and thought, 'I wonder if I can sit there?' Well, the answer is no, and we're here to tell you why.
The simple answer is, it would technically be stealing. If you've paid for a coach ticket, it would not be fair to other customers if you were given a first-class seat.
Recently, United Airlines tweeted in reply to the question, "What's the point of empty seats if they can't be filled and your customers can't have a better experience?" The airline pointed out that, "if you were to purchase a Toyota, you would not be able to drive off with a Lexus because it was empty."
If the attendants find you sitting in a seat that you didn't pay for, they have every right to ask you to move. There are even stories of people getting arrested over the first-class seat switch debacle.