Nonsense? To the contrary, this is the answer to OPs question “What would diffracted light waves look like if they did change their energy at diffraction?” The light waves would change frequency if they lost or gained energy as the amount of energy carried by a photon is directly proportional to it’s electromagnetic frequency and is inversely proportional to it’s wavelength. So, Photon plus or minus energy equals frequency and wavelength change. Sensible, no?