Just because there is an empty seat in business class doesn't give flight attendants the supreme authority to allow you to set up shop there. If you cop an attitude about this, your flight attendant will not be pleased.
“Flight attendants are not the ones who have the authority to provide upgrades,” writes Sullivan. “That usually is the job of the gate agents or other customer service reps. If a flight attendant will not let an economy passenger move to an empty business class seat, it is because she or he is not allowed to do so.”
Remedy: If you didn't book ahead online – where you can often choose your precise seat – and if you don't have a serious reason why you need to change your seat (versus want to change your seat), get over it. You’re pretty much stuck.