So why do you need a Full Frame chip? Particularly one in a camera that delivers 4K as does the FS7. The answer is that it delivers “better” pictures but exactly what does that mean? It is still a Bayer style single plane sensor. When an FS7 takes a 4K picture it will take 2K green cells and 1K each red and blue. That does not seem to offer the prospect of a good picture however Mr Bayer’s algorithm attempts to assign a best colour to each pixel. So it is not obvious that a Bayer sensor was used – it looks full colour. With the Full Frame 6K chip it offers to do a much better job of this. What I would like to see is a engineering comparison of the two.
Back to Full Frame, at face value you have to buy a new set of full frame lenses. However one of the Sony Kit lenses (for the FS7) was delivered as full frame before there was any need to. Similarly any E-Mount lenses you have for the A7S series are full frame. You may find you have a set of lenses for the FX9 already.
One of the problems of 4K sensors is that it can take a large and expensive zoom lens. These are often quite a bit more expensive than the camera. A Super 35 E mount zoom lens will still work on the FX9 but the camera may have to be down rated to no better performance than the FS7. At the BBC NHU session they said they have had this problem for a while and have been using adaptors to put the HD lenses on UHD cameras. Perhaps a market in adaptors for the full frame cameras.