First, let me say those peopel were rude a@#holes. You didn't deserve that.
However, I will say that if theatres are going to sell premium seats at exorbitant prices, then I don't feel folks from cheap seats should be plunking down in them unless they are asked to. If I have paid $250 bucks (sometimes I have) for first three center rows, then I don't think it really fair that someone just helps themselves.
That being said, I'd never reprimand someone but I would inquire if they were huge and blocked my view.
It's like airlines. Jetblue won't let you move to an extra legroom seat if the plane is full and those seats are all empty because you didn't pay the premium.
I've never moved seats unless and usher has asked me, which they have on occassion.
"The sexual energy between the mother and son really concerns me!"-random woman behind me at Next to Normal
"I want to meet him after and bang him!"-random woman who exposed her breasts at Rock of Ages, referring to James Carpinello