The only flight upgrade I've had was on a UA flight from SF to SYD, I was a student in HI, and was flying home on a "buddy pass" basically standby, a friend worked for UA, and I flew from HI to SF to SYD for I think it was $300USD back in 1998.
After flying Y to SF, I had an all day layover checking out Alcatraz, and the piers. returned to the airport for my flight, I check in with the desk around 6pm for a 11pm flight I think it was. Boarding time comes, so i'm at the gate waiting for my name to be called to get a boarding pass (I had no idea how things worked so didn't know what to expect.) I had my hopes up to get something towards the front of the plane, but since other people had been called up, I resigned to hoping that I would get an empty row down the back to lay down.
After what seemed like forever for young me, my name was called and I got my boarding pass, and to my delight and surprise, it was business class on the 747...
Now my memory isn't the clearest, but the seats weren't the fanciest, or did anything great, but there was more room than in Y. I don't remember the finer details, or what the food was like, but something I do remember, Before landing in SYD, the lady next to me, she had a glass of what I think was champagne, and she was asleep, she wakes up and accidentally knocks the glass over onto me, and down the side of the seat. Flight attendant is called, and she right away gets me to get out of the seat, exclaiming that she doesn't want me to get electrocuted...!! Since we were landing soon, all they did was stuff a bunch of blankets down the side of the seat so I wasn't sitting in a wet seat. Apart from that, Business class on basically a free ticket was nice.