The main downside to switching the hash function is that, when explaining why developers should stop worrying about hash conflicts, we'll need to calculate a new analogy to replace the standard, 180 bit "every member of your programming team being attacked and killed by wolves in unrelated incidents on the same night" scenario.
That analogy presumes that the hash function's output is uniformly random; when you know how to manipulate it s.t. its output is not random, then obviously it doesn't hold.
The question of accidental collisions is still relevant, even with SHA-256, and the answer is still the same: it's so vanishingly improbable that it is assumed to be impossible.
