Did it myself couple of weeks back on Lufthansa flight from FAM to EDI. I was in the middle seat of the first economy row. It was a very full flight but when the doors were closed the row in front, business class, was completely empty.
As there was three of us in my row I asked the guy to my right if he wanted to move forward and give us all a bit more room. Being German he was mortified at the thought so I skipped in myself figuring that the worst that could happen would be that they relegate me back to my own seat.
CC never turned a hair but, for my own conscience, I didn’t take the meal etc. that was offered as all I wanted was a bit more elbow room and space to set up the laptop.