I tried this exact move a few months ago.
I was sitting in the upper deck and saw a huuuge empty area down towards the top/back of the lower deck, so We went down there after half time, assuming since it was still empty, it would be fair game.
We hadn't been sitting there for 10 minutes when this pleasant grey haired gentleman asks "Tickets please" and I say "ok, you caught us." He says "Ya, we look for people doing that down here, and we're going to be looking for people all game long, You'll have to go back to your regular seats". So we headed back up up up the stairs.
I watched the section off and on for the rest of the game, and I saw that guy kick maybe 20 people out of there after he got us.
Thinking back, that was the last time I went to a game. I haven't really been interested in going back to arco since that day, though I doubt its really correlated with the seat swap. Probably had more to do with what a stinker the game was and how unpleasant a lot of the fans were. I still remember the guy 4 rows from the top of the arena shouting "Go back to New York Napier". sheesh.