not an easy task as you think, because what miners can do then? get rid of their whole 300 peta hash and start all over again...no way it would be one of the biggest waste of money...
it would mean the death of bitcoin as we know it now
What else could you possibly do..... If sha256 was broken the miners would be worthless instantly bitcoin would HAVE to change the algo. Once one can solve private keys from public it's gg for SHA256.
GPU's would rise again from the ashes.
it should be a gradually change if they really need to change because of quantum computer(are you ttalking about this right?), so it must be done many years before it happen
the problem is that it can be build secretly by some huge company, then bitcoin will be indeed screwed