I also don't see how these people could be accused of endangering the aircraft for changing their class of seat (assuming that's all they did) Maybe they could be charged with deception in the same way as someone who buys a second class ticket on a train who then moves to 1st and refuses to pay the extra.
Despite what Getoutofmygalley says not every instruction from cabin crew can be automatically construed as legal order from the commander of the aircraft, this is especially true if someone were being prosecuted for endangering the aircraft because they moved to a different seat and then refused to return to their correctly assigned seat after the 'order' was issued. We all know that cabin crew don't give a damn about PAX changing seats as long as it's within the same class. Any first year lawyer would be able to successfully argue this if his client were accused of endangering an aircraft because he changed seats.
Obviously I'm not condoning what these PAX did nor am I even suggesting that anyone should even consider ignoring a request to do something from a member of the cabin crew, I normally work on the assumption that if they tell me to do something then it's for a good reason and I'll do it because they know what they're doing and they know an awful lot more about aircraft than I do.