I've been 100% successful using companion upgrades at the airport. If I see someone who looks worth (little old lady, etc), I'll ask if they're flying FC or Coach. Then if they're interested, I'll go up and get them UGed. I don't do it a lot, but have never been turned down yet. The last time I did it, MSP-PHX, the flight ended up being oversold, so the lady was doubly glad to ride in the front. Her first time ever.