You can ask but it doesn't happen often
A guy infront of me on a flight got offered an upgrade to business class to accomodate a woman who had plaster casts on her arms when he was clearly unhappy at moving
We found it quite funny that they didn't just upgrade the poor woman who had the injuries lol, she had a face like a smacked **** when she heard them offer him the upgrade

If you look smart(ish), friendly and especially have frequent flyer cards or similiar there is a small chance, if none of those apply then your not gonna have much luck.
The only time I've been upgraded I didn't ask I think the girl on the desk just liked me.


(Virgin Atlantic)
My Father always travel in posh looking suites and pretty much gets upgraded every time on a long haul