I will usually trade "equivalent" seats except when some agressive joker is already setting in my assigned seat and loudly sez "I didnt think you would mind if I sat here because........(or whatever) and you can take my empty seat over there". At that time I say "Sorry, I really prefer to sit in my the assigned seat I paid for and maybe you can ask someone else to trade seats with BEFORE you sit in it".

MisterNice