Reckless endangerment of an aircraft
is a typical stop-gap phrase in legalese. It would take a more than average lawyer to convince a judge of the "endangerment" bit and even much more about the "reckless" bit.
But such is not even the point. The original question was a bit ambiguous on its subject matter (was it about "people on board the plane changing from one seat to another" or about "replacing one piece of seat furniture for another" ? ) but that has been dealt with; the original question was however very clearly "is it legal?" and has been duly answered with "none see any indication of it being illegal". QED. Whether it is a good/wise/defendable practice is quite another discussion, you seem partial to it, and that is entirely up to yourself.