I've never seen that happen. I can think of more than a few times where I've given my FC seat to someone in Y so that I could sit next to a friend of co-worker. I couldn't imagine a FA telling me that she needed to give my seat to another person. That would be like trying to give your seat to a soldier in uniform only to be told that if you're going to give up your FC seat it needs to go to a Global/MM1K, etc.