Suppose that I'm traveling with a family member, but he/she doesn't get an upgrade while I don't. Is it ok for me to trade my seat with him/her for the entire flight?

I know it would be pretty unacceptable for just part of the flight, but for the whole flight, it should be ok?

How about for someone you just want to give your seat to, ie. not a family member or someone traveling with you? For example, what if I just wanted to give a random someone in coach my seat up front out of kindness?