Lewis' solution to these difficulties, is to argue for a distinction between "a real moral advance" and a "real moral innovation." He argues that Christ's Golden Rule is an advance from something such as Confucius' "Do not do to others what you would not like them to do to you." However, he argues that Nietzsche's ethic is an "innovation" which does not fit within Natural Law because it rejects its classically acknowledged contents.
But the Nietzschean ethic can be accepted only if we are ready to scrap traditional morals as a mere error and then to put ourselves in a position where we can find no ground for any value judgements at all. It is the difference between a man who says to us: 'You like your vegetables moderately fresh; why not grow your own and have them perfectly fresh?' and a man who says, 'Throw away that loaf and try eating bricks and centipedes instead.'And so the bigger picture answer offered by Lewis - in a very barebones form - is to suggest that, of course, the Natural Law is objective and fixed, but that its application to each society is, to a certain degree, flexible (within a certain allowable range). To step outside of the range is to innovate. Those who stay within the range are making progress.
Modifications and advances in Natural Law are found throughout almost all societies. In fact, all societies which believed in objective right and wrong do have some variations in their laws (especially, Calvin points out, in each society's sanctions against lawbreakers), but these are differences of degree and not contradictions. Wherever contradictions exist, someone has legislated against the Natural Law. Humanity needs moral advances, not Nietzsche-esque innovations.