If you ask, they'll almost always say yes. Something like, "Hey, I noticed the flight is mostly empty. Would it be possible for me to get a complimentary upgrade to one of the empty first class seats?"
Right now I'm in premium economy as for some reason the ticket was cheaper than economy.
Realstically speaking there is virtually no difference in service for them.
Assuming that I get the usual economy food and I'm really the only person on the flight.
2. Does it cost them significantly more to seat you in a different location? I'm asking, I genuinely don't know.