[lambdaone]

Even though it might seem unlikely, you absolutely can't eliminate the possibility of planes falling out of the air unless you have thoroughly audited and tested all their systems in advance. Not having planes in the air at the exact moment of rollover - just in case - might be a good idea too. Similarly financial markets and other key infrastructure.

13 years is plenty of time to make a start on all of this, and will allow the costs to be spread over that time period without undue hurry. It's also long enough that it will be possible for much software to be allowed to reach end-of-life without being audited, while applying rigorous testing to new software. Procrastination, though, is not a good option.