I frequently give mrs tvtd my upgrades. It’s never been a problem, with the exception of our last trip, when the mrs brought back the fruit cup from the breakfast offering. The FAs were visually upset when I physically returned the cup (a bit surprising as they would not pick it up during cabin service). All that said, I would not trade seats during the flight, nor would I ask if it was okay to do so, unless there was some unforeseen circumstance that the FAs would more than likely know about if that situation were to occur. Of all the things security related, PAX switching seats between classes probably rates pretty high on their list.