Found this part interesting:
The challenge, she said, was to switch the systems progressively so that passengers would not notice. Ms. Wise, who has a doctorate in applied mathematics, devised a low-tech solution: she set up a timeline of the steps that had to be performed by pinning colored Post-it notes on the wall of a conference room.
A major switch happened when the new airline canceled all Northwest’s bookings and transferred them to newly created Delta flights in January 2010. It required computer engineers to perform 8,856 separate steps stretched out over several days.