Can it be done? Is it legal? Is it stupid? Three very different questions with three possibly different answers. I have changed seat from front to back and back to front in a PA28 - when solo.
It was my QXC, I was only 17 (this is many, many years ago). I had flown the route previously and had flung my chart onto the passenger seat, content I knew the way and sitting back to enjoy the scenery. As I approached my first stop on the route ATC asked me to report at a named reporting point of which I was unaware. I reached for the chart but it had vanished. After a search it became clear that it had fallen into the rear of the aircraft. I unstrapped and slid between the seats to retrieve it. Sat momentarily in the rear seat I looked up at the very odd view of one of the world's first and least safe pilotless aircraft. With the chart in hand I resumed my place in the captain's seat which I was clearly unsuited for. Being 17 brings with it a feeling of invincibility which is entirely illusory but which we often get away with. If I had got stuck and crashed it would have been my own fault but would have been an interesting one for the AAIB.
So the answers for this scenario are yes, no and f*** yeah. The answers for the original question may differ slightly but in aviation if there is any doubt, there is no doubt. It might be a story to tell in thirty-odd years time or you might never get to tell the story with an AAIB inspector writing it for you. Is that a chance worth taking?