A couple of years ago I was fortunate enough to be upgraded at the gate from Club World to First. Unlike other previous upgrades, my boarding pass was not re-printed to show the new assigned seat (the person at the gate simply used a pen to write my new seat number on my boarding pass stub). I did wonder if the crew onboard would think that I'd altered the pass myself....I assume such an obvious scam has been tried and that the crew know how many passengers they should have in the premium cabins, their names and where they should be sitting.