I have given my "Y" seat up at the last second, while inside the plane, almost ready to take off. As soon as I got up, and grabbed my bag from the over-head compartment and started to walk off the plane, the steward said, "No, we've got a seat for you..." and escorted me to first. Since this was a NYC to SFO, the upgrade was most welcome.

NEVER underestimate the power of generosity!

Also, of course you're not suppose to do this and I certainly don't recommend it, or condone it, but I have in the past, sat down in First and began conversing with a fellow passengers and ended up flying the whole trip in first (lax--->bankok and return) and others. But again, unless you're willing to accept the consequences, I wouldn't do this if I were you.

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g'day