They transplanted his brain to a chimp.

He is hard to look at.

It's all in the teeth! 1983 hawking had discolored nubs for bottom-front teeth, 2015 hawking photos show recessed gums holding stained full-teeth on bottom. So,, unless he had pre-stained caps invisibly installed onto his nubs, it's not the same man.

It was an inside job. The science community was mad that he was about to publish that he was wrong on the rotation direction of a black hole.

I think he is on enzymes and other genetic altering stuff. Even just plain fungal infections can chabge your pehnotypucal traits over time. In the 2007 comparison of the Zero G Hawking, notice the crinkle between the eybrowns along with a few other wrinkles... While astounding/baffling, I submit this is the same man. How they pulled this off, I can't be sure.