Despite some of the oversimplified solutions earlier in this thread, "switching" to a new hash means creating a new (possibly derived from the existing one) protocol and an entirely new network (possibly based on a genesis block offering BitCoin funds to the SHA-256 addresses that had them outstanding). Back in 2010, there was only a single client, and reinventing everything may have seemed like a simple solution. But beginning with 2011, we are starting to see alternative implementations of BitCoin, and by the time SHA-256 is broken, we will no doubt have many various possibilities. If SHA-3 is due out soon, it might be early enough for all the implementors to agree on reworking the network around it...