In all likelyhood we could switch to a new hashing algo before any real damage were done.  Remember, numerous weaknesses were found in MD5.  granted MD5 wasn't broken outright, but several algo weaknesses found significantly lowered the strength from 128 bit to something like 112 bit.  The world abandoned MD5 for anything critical and everything was fine.  Sha256, likely has no silver bullet style weakness.  At worst I think one day we wake up and some dude found a big prime common factor or somehting and the crypto strength comes into question.  On day 1 of the exploit Gavin patches to a new algo.