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The question you should be asking is not how big a change this is in the world, but how big a setback it this for the protagonist and how does it impact his/her moral arc. The basic structure of story is of an escalating series of challenges leading to a revelatory moral climax in which the character makes some life-altering decision and then lives out that decision. 

So the question would be, how do you fit such a change into the moral arc of the protagonist? Since it occurs only 1/3 into the story, how are you going to top it to present a yet more difficult challenge to the protagonist? But that means a challenge that is more morally difficult, not one that is more physically difficult. 

If you can answer those questions satisfactorily, then go for it. It really does not matter how big the challenge is physically. It matters how great it is morally. A far more subtle change could present a far greater moral challenge. As long as the moral shape of the story is maintained, you should not have a problem. But if the moral shape of the story is distorted, then the story is not going to work.