I frown on such things. I have been upgraded courtesy of the airline before, upgraded myself on miles, upgraded by paying a little extra at the airport and been upgraded after being rebooked onto different flights, but I have never been so brazen to sneak into a class I shouldn't be in.
I'd be mortified, and I'd be one of those raising it with the c/c. There are instances where I can understand it (sportsmen being injured and flying economy - and being moved to business for their comfort - no-one wants one of those to die of DVT related causes).
S.