I presume they've considered this, but this seems like a dangerous approach. How will they decide which change occurs, how far advance will it be decided, and how will they keep this decision secret? Think of the potential advantage to a party who uses foreknowledge of the exact change to have ASIC's ready at the official announcement. If done on a small scale, it's possible that no one would ever know. If done on a large scale, it would be obvious after the fact that a leak had occurred, but the repercussions would likely destroy the currency.