I think that is one of bitcoin's best features. the difficulty is > 800k so the hardware infrastructure is solidly in place. all that is required is an online vote with > 50% agreeing to the new client. would result in 2 (or more) alternative currencies, but no worries, the new miners and conversion exchanges would be up in days with personal API bots doing most of the trading/converting. it would all be over before any widespread exploit of SHA-256 could disrupt it significantly.