On my CDG-ATL flight last week, people swapped seats in eco without asking and warning the crew. I guess that they didn't have the choice for their seats and decided to move on their own.
Unfortunately, when the crew had to distribute custom meals, it really was a mess. Giving custom meals before serving the whole cabin one after take-off should have been a common routine for them and it took them twice the time, they were quite upset. They also explained that in terms of security, everyone has to seat where one has been assigned to.