Swapping seats during the flight can abuse the privilege. Say person 1 gets loaded on free drinks and thinks that their friend in coach can sit in their fc seat and do the same for the rest of the journey. Constant swapping can also appear to be a very visible attempt to let the FAs know that someone in coach wants to sit up front. It's not a problem to swap prior to takeoff, but I wouldn't change again during the flight unless it has something to do with one person wanting the meal or better sleep accomodations on a long flight or a redeye. I don't think the FAs would mind if it doesn't look like anyone is abusing fc privileges. I once gave up my fc seat for someone I didn't know in a full leg cast, but the FA brought me fc drinks back to me in coach for being so gracious. We should all offer our fc seats sometimes to those wide-eyed in wonder seldom flyers that may never otherwise experience it. What we take for granted, they will be telling their friends about for months.