I wouldn't try it. On long-haul international flights especially the FAs are going to be very vigilant about anyone shifting seats. Not just because of the rules that others have explained but also because of security. I flew PEK-SEA on DL recently in J. A family of 3 put their teenage son in J and they were in E+ (I think he had special needs), and they couldn't come visit him -- the FAs and purser did a fabulous job of checking on him though. I think it's a little hyper-paranoid myself but welcome to our modern world.

Also, the E+ area on that DL flight was curtained off from the J cabin and the main Y cabin and had plenty of empty seats so it was clear they weren't bumping anyone up.