The only time I have ever in my life been upgraded was on Syrian Arab airlines (many years ago!) on a lightly loaded 747SP where the stewardess asked me if I, and the couple of people I was with, would mind moving from economy to the business section near the front of the aircraft "in order to balance the plane for take off"!
I was surprised as I'm not that big a person! But as I recall I didn't object at the time.