I've seen that a lot on the LAX-PVG or PVG-LAX flights. One of the reasons for this, I think, is a lot of tour groups get booked together into E- at a super discounted fare. So the E- seats tend to fill up pretty quickly.
When this happens, those of us in E+ will usually have a betting pool to see how many E- seat poachers try to come into E+. Eventually it becomes annoying though, as the FA's repeat the announcement not to move to E+ in both English & Mandarin over and over. One time, from LAX-PVG, a passenger decided if the FA wasn't going to let him jump into empty E+ row for free, he thought he could get it by "suddenly" feeling very sick and the FA would give him sympathy to move there. This only resulted in the FA making an announcement asking for a doctor (there was none) and then the Captain announcing he was diverting the flight to HNL due to the sick passenger (as we were 1/2 way across the Pacific). You wouldn't believe how much yelling there was at this guy in Mandarin. I don't understand the language, but I'm pretty sure everybody was telling him to stop pretending. Well about 10 minutes later, the passenger had a "miraculous" recovery and went back to his seat. The flight got to PVG only a few minutes late.