United Airlines is set to upgrade its reservation system. Here’s what that means for travelers.

United Airlines is planning a major technology update that will take down its online reservation system early Wednesday.

The carrier is urging passengers with flights scheduled to depart on Wednesday to check in via United’s website or mobile app on Tuesday to avoid encountering problems. United expects the outage to last roughly four and a half hours, beginning Wednesday at 2:30 a.m. EDT and lasting until 6 a.m. EDT.

An alert on United’s homepage informs travelers that it is “conducting a planned technology upgrade” and that many of its web and app services will not be available during that time, including for employees of the airline. 

United is modernizing its aging mainframe flight reservation system, known as Shares. The upgrade requires the airline to transfer reservation data from its current data center in North Carolina to a more high-tech facility in Chicago that the airline said will improve reliability.

Here’s what United customers should know about United’s reservation system upgrade. 

What if I am flying on Wednesday?

United’s flight schedule will be temporarily unavailable starting early Wednesday morning. Customers also won’t be able to make new reservations, cancel bookings, check in for flights or retrieve their itineraries. 

As such, the airline is urging anyone scheduled to fly on Wednesday morning to check in on Tuesday. That is also when passengers should check bags, which fliers can typically do up to four hours before departure.

The reservation system refresh has been planned for some time, with United canceling impacted flights months ago. The airline notified customers of the changes, affecting roughly 600 flights, at the time.  

Customers can still book, complete check-in or make changes to their trips prior to Feb. 4 at 2:30 a.m. EDT, or once the data transfer is completed. 

What could go wrong?

United has rehearsed the data migration process, so things should go smoothly, the airline told CBS News. Travel experts expect relatively little disruption.

“Don’t freak out if you can’t search for flights in the middle of the night,” Scott Keyes, founder and CEO of flight deals website Going.com, told CBS News. “It will come back online soon.’

Of course, if the upgrade doesn’t go as planned, it could spell headaches for customers. Roughly a decade ago, Southwest Airlines suffered a computer outage that dragged on for days, leading to hundreds of flight cancellations and thousands more delays.