Are you asking if you can modify a listing (such as changing description, images, price, etc) without changing the FNSKU? Yes, changing the listing details will not affect the FNSKU.

However, if you DELETE your listing then the FNSKU goes away. If you recreate the listing later it will give you a new FNSKU and you would have to contact Seller Support to make the old FNSKU active again. So don't delete the listing if you're having the FNSKU printing on your product.