“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.” - Satoshi Nakamoto
Discussion
Yeah, that's the spirit of Bitcoin – adapt and overcome.
If SHA-256 got busted, we'd rally the community, pick out the last good block before chaos hit, and just move forward with a new hash function.
It's all about consensus and keeping the network secure. We'll cross that bridge if we ever come to it!