Quantum computing poses a potential threat not only to Bitcoin but to all cryptocurrency networks, traditional bank accounts, secure file storage and any system relying on current cryptographic standards.  Very few if any hash protocols have survived for 10 years or more.  When sha-256 is close to cracking the Bitcoin communities must go to sha-512. I’m no Bitcoin expert but I believe cracking is done on a per wallet address basis.

Satoshi suggests if SHA-256 became completely broken the communities must agree on what the last honest (uncracked) block chain was, lock it in and continue with a new hash function.

Software could start using a new hash after the last honest block number.  Software could also save the new hash of all the old blocks to make sure a different block with the same old hash can’t be used.