I am just a non-regular SLF, but rather tall (approx. 2 m), and accepting the very limited space in economy. I will try to get a seat with some extra space, eg emergency exit, but usually this is impossible, even when trying to check in through internet 30 hrs prior to departure. Flying in an airliner is an inconvenience to me, which I will try to avoid if possible. Nevertheless, sometimes it is unavoidable and I accept the consequences of an affordable (to me) fare. As the Dutch are amongst the tallest people in the world, it strikes me as odd that even KLM offers very, very little space. I do have little faith in walking away unharmed in case the a/c would crash.
Still, obviously, I would not sneak into business class or try to talk myself into it with excuses. I will ask the crew if it is ok to switch seats within economy class, if there is a more spacious seat available.
Only once I made a mistake, not aware of the fact that the first row in economy actually was an upgraded class. It was only a small aircraft and the first row of economy was empty, me - somewhere in the back - having no leg space whatsoever. So I asked the cc and got a very annoyed answer that it was an upgrade and that I was not allowed to sit there unless I paid for it - I assume I was not the first to pop that question. It left me a bit uncomfortable, since it was impossible for the passengers to see they were upgraded seats.