Personally I don't think bleaching is as big of a problem as acidification. The natural selection process they are attempting to speed up happens naturally anyways. Corals bleach, then the more robust ones which didn't bleach overgrow them and the reef lives on.

Acidification is much worse, because you can't build calcium carbonate without carbonate ions, and acidification lowers the quantity of carbonate ions in the water. There is no natural selection for this, you can't build coral skeleton without the proper building blocks. It just makes all corals grow much more slowly, and thus when negative events happen it takes the reef much longer to recover. If the pH goes down low enough, at some point corals just can't build their structure anymore, which would be the end of the coral reefs as we know them.

And the only way to prevent ocean acidification is to lower the amount of co2 in the atmosphere, which isn't happening and wont happen for a very long time. It is going to be a tough century for the coral reefs.

Not-fun fact: The atmosphere is around 0.04% CO2, yet dissolved CO2 (either as CO2 or carbonic acid) is by far the largest dissolved gas in the ocean because of the reaction which forms carbonic acid. See the graph of atmospheric co2 vs ocean pH [1] to see how large the impact is.

1: http://ocean.si.edu/ocean-photos/ocean-acidification-graph