And the analogy is a bad one, too. I think it's trying to make the task sound more difficult than it is. If we stick with the airplane analogy, maybe this is

And the analogy is a bad one, too. I think it's trying to make the task sound more difficult than it is. If we stick with the airplane analogy, maybe this is better: it's like incrementally converting an older model jet engine to a newer model jet engine in between flights. Every incremental version must work correctly, which isn't easy, but the upgrades are definitely performed on the ground.