No player can change his playing style.
It sounds to me like you think brains are hard-wired, which they are not. Most human brains are not blocks of wood; they can reorganize and adapt; that's what is meant by 'brain plasticity' in neuroscience. Tennis players have to make stylistic adjustments according to surface and every year they do exactly that. Ask any tennis player about the adjustments he has to make when transitioning from clay to grass, or from hard court to clay. They don't just change their shoes, they adjust their playing styles somewhat. Nobody says, well geez, why don't we have the same surface like all the time? Or, why is this surface so aggressive, don't the organizers care about my health? No, they just deal with it and make the necessary adjustments.
Aren't we talking about one of the greatest players of all time here? A legend in the sport? Besides, Nadal doesn't have to rediscover the wheel but he can apply a different approach/style to different surfaces. His brain already comes with a wealth of experience on the 'how-to' of playing on different surfaces. Nadal did do a little bit of that at the 2010 USO if you recall. He was cracking a bigger serve and trying to finish the points earlier, more aggressively. Again, are we saying that his brain has become petrified, or that he suddenly lacks the skill set and creativity to do this? (I don't think so.)
Or is he maybe still smarting from the fact that a relative no-name just beat him in a clay final and looking for ways to vent his frustrations (injuries, no-name losses, etc.)?