At long last, Southwest Airlines will migrate its domestic operations to the Amadeus Altéa passenger services system on May 9.

The carrier moved its international flying, inherited in the 2011 acquisition of AirTran, to Altéa three years ago.

Southwest carries more domestic passengers than any other US airline, so even without the international flights the move is no small undertaking, and the airline has approached it in phases.

It also took a page from the successful transition of US Airways to American Airlines’ system in 2015, which employed a “drain-down” process.

Last December, Southwest began booking new reservations on flights slated to depart on or after May 9 in the Altéa system.

That means that very few reservations will have to be moved to the new system on the cutover date.

Migrations tend to range from glitchy to disastrous, but the drain-down method, the three-year stint with Altéa for international flights and Amadeus’ long experience with migrations should give Southwest an edge.

Southwest also has been preparing for the human side of the transition. Reservationists have to be trained, but because the carrier is adopting the full Altéa suite, it will also have a new departure control system, which requires retraining all airport employees.

In all, about 20,000 employees have undergone training on the new system. “It’s been all hands on deck,” CEO Gary Kelly said during the carrier’s recent earnings call.

Southwest has long labored under archaic technology. Its passenger services system is based on the Cowboy PSS developed by Braniff Airways, a carrier that ceased operations in 1983.

Sabre has hosted the system for decades and made various modifications, but Southwest has been inhibited by its ancient technology in several ways.

The move to Altéa will enable a number of changes to its operations, such as code-sharing ability, seat assignments, improved revenue management, red-eye flights and the implementation of checked bag fees.

Southwest has said, however, that just because it can charge bag fees doesn’t mean it will. “Bags Fly Free” has been the airline’s mantra for nearly a decade.