NW still allows upgrades on free tickets for its top travelers. And many carriers will let you upgrade by paying the difference between Y and F.

Meanwhile I was on a flight the other day where somebody from Y decided to sit in F. When confronted by the flight attendant he said that he had upgraded on the ground. He couldn't find his boarding pass and the manifest showed him in Y. They asked that he give them his credit card or move back to 41F. He gave them his card. I do not know how much they charged him. The gumption!