Been upgraded 3 times - twice with BA, once with VS. Both BA ones were for operational reasons I suspect (didn't ask for it) but gratefully accepted! The VS one, I was in a PE seat, someone came up to me with an identical boarding pass from a connecting flight. Looks like there was an IT hiccup and she was travelling as a carer for the person next to me. As it was a busy flight, I said to the CC, I'd happily move to accommodate the person, but as it was a busy flight, if I needed to move cabin, I'd prefer to move forwards, not backwards.
Five mins later I was seated in upper class, with the furious passenger who ended up in my seat shooting evils at me! :-)
When I first started flying 25yrs ago, it was a rumour that if you flew smartly dressed on the Virgin Atlantic service when using it with MOD, you'd get an upgrade. I was young and believed it, and thats why I flew in suit a couple of times, before realising that it was nonsense!