The takeway IMO is that the reason their transition was so seamless is because they took the time to do it 'the right way' - that is: set up the new system in parallel, duplicating the reads and writes, and verifying that things looked good every step of the way.

That gives you not only a long window to test and validate, but also the option to rollback if things suddenly don't look so good with the new system. It seems like people sometimes get impatient and want to do a 'big switchover' with insufficient production testing and no real rollback plan, but in my experience that's almost never a good idea.