S-works, I think we've all caught on to the fact that you have a strong opinion about this. But as others have mentioned, the question was not whether it was wise or not. I fully agree that in most cases (almost every case) it would not be a smart thing to do, but I could also think of a situation where changing seats might enable someone to successfully complete a flight while staying in your seat could lead to a crash. Without going into specifics, what I'm trying to say is that it is all very good to expect pilots to follow the rules at all times, but when someone gets in a bind, we also expect that pilot to use of of his/her creativity to get him/herself out of that bind. If that includes swapping seats, I wouldn't want that pilot to get caught up in the mantra of 'the law forbids this... I must not do it'.
After the fact it is easy to be judgmental, so we'll be just as quick in judging someone when an in-flight seat swap leads to a crash, as we'll be in congratulating someone when an in-flight seat swap saves the airplane and its crew/passengers. The line between recklessly endangering the flight and being the hero of the day is very thin, and is very much dependant on the eventual outcome. I think it is a good thing that the law doesn't specifically forbid the practice, apart from the CAA's rule about non-EASA types. Yet I won't go around advocating it as a serious option, don't worry about that. The law provides us with a defined playing field within which we can operate aircraft, but not all edges of that playing field are hard black-or-white borders. The topic starter asked us a question about one of these borders and we've done our best to figure out if this particular edge is black-white or whether there may be a bit of a grey area.
Funnily enough, we've been talking about this from a UK/Europe perspective and no-one has caught on to the fact that the topic starter is in Auckland (according to the profile info bit) and all of what we've found out may be completely irrelevant. I don't expect the rules to be all that different, but still....