Re: Major Meltdown
Here's an answer to a similar question about how to recover from a major meltdown.https://www.bitcoin.org/smf/index.php?topic=191.msg1585#msg1585
If SHA-256 became completely broken, I think we could come to some agreement about what the honest block chain was before the trouble started, lock that in and continue from there with a new hash function.
If the hash breakdown came gradually, we could transition to a new hash in an orderly way. The software would be programmed to start using a new hash after a certain block number. Everyone would have to upgrade by that time. The software could save the new hash of all the old blocks to make sure a different block with the same old hash can't be used.