Switching reservation systems is a crucial part of an airline merger, and a scary proposition. If it doesn't go as planned, it can be traumatic for the carriers and their customers. Delta Air Lines in 201o had some glitches when absorbing Northwest's reservations, but overall the migration wasn't too disruptive. It was a very different story at United two years later.
American Airlines now is targeting as early as October to switch US Airways from the HP Shares system to its Sabre . . .