I've seen a lot of people try but I've never been aware of anyone actually getting away with it.
A guy tried it on a flight I was on from MUN-SXB last week, there were only two of us in business class and as soon as the door was closed a guy move with lightening speed into the row behind me, the CC immediately noticed and told him to go back into economy class. It was all a bit pointless because it was a 40 minute flight on a CRJ200 and there were only about 15 people in economy. All the seats are the same, no-one was sat next to anyone else in either class and there was hardly a lot of difference in the food.