. . . Even if SHA-256 is partially broken (say easier to find hashes, but not fully broken) . . .
. . . If SHA-256 is ever broken, it won't matter much what the ASIC miners had planned.
If it is only partially broken, we'd just see the difficulty increase substantially.
And in a way that ASICs likely couldn't compete with anyway, since they can't be altered to take advantage of any shortcuts found; whereas GPU miners most certainly could. I'd say it's almost a certainty that if any portion of SHA-256 appears to be at risk, there will be little resistance from the mining community.