At United Airlines, they called it the “cutover.” It was the final and most difficult piece of the puzzle in the merger with Continental Airlines, and it involved combining two complex passenger reservations systems.

But some United passengers referred to what happened in March as something else: chaos.

They complained about delayed flights, sluggish customer service response times and rude treatment by overwhelmed ticket agents struggling to learn a new computer system.

A closer look at the cutover and its repercussions suggests that no airline is immune to a systems failure that could affect your next flight and that beyond a common-sense strategy or two, passengers can’t do much to prepare for a meltdown.

On another level, United’s switch is also a case study in how careful planning by the airline’s customer service team averted a disaster that could have inconvenienced even more passengers.

The lead players in United’s IT drama are two reservations systems that handle functions from ticketing to loyalty programs. United’s was called Apollo; Continental’s was Shares. United chose to use Shares shortly after merging with Continental, and late on the evening of March 2, almost a year and a half after the consolidation became official, it completed the process of combining Apollo and Shares by copying the data on both systems, backing it up and then consolidating it.

That part of the cutover went relatively smoothly. But on March 3, United customers awoke to a new Web site, and the old Continental site now pointed to United.com. And airport agents powering up their workstations were greeted by a Shares log-in screen, a system that half of them hadn’t used in a real-world setting.

Adding to the confusion, United made other, unrelated policy changes at about the same time. Those included tightening several rules for its frequent fliers, who also tend to be its most vocal customers. Lower-tier elite-level passengers could no longer get upgraded to United’s premium economy class when they reserved their tickets; they had to wait until the day of their flight to secure their Economy Plus seat assignments. Their free-checked-baggage allowance was also cut from two bags to one, generating more complaints to United’s call centers and straining the airline’s resources.

Shares began to slow under the weight of the extra transaction requests, leading to more timeouts and system freezes than United had anticipated.

Call volumes surged from 1.5 million the week before the cutover to 2 million the week of the switch, exceeding what managers had planned for by 10 percent. “Handle” times — the length of time it takes to resolve a customer inquiry — jumped 120 percent. Answering inquiries took an average of 20 minutes as many agents wrestled with their new Shares interface. That, in turn, lengthened the call wait queue. Some customers hung up after spending what they said was hours on hold.

John Buckholz, a planning manager in Ogdensburg, N.Y., spent more than three hours on the phone trying to cash in a gift certificate, which involves more steps and usually takes longer on Shares than it did on Apollo. “There was no way to talk to someone who had any power to do anything,” Buckholz complained. “Anyone I talked to was extremely short with me and just told me there was nothing I could do.”

Sergei Shevchuk, a research scientist in San Francisco who tried to phone the airline during the cutover, also grew increasingly frustrated. He’d canceled a ticket just before the switch, and when he called to find out about his refund, agents offered conflicting answers, first saying that his ticket wasn’t refundable and then that they couldn’t find his reservation. “No one there seemed to be able to locate one of the tickets,” he said.

Kathy Talcott, a United passenger who’d been waiting since January for a promised refund, received an e-mail from a customer service manager who blamed the cutover for the delay. “Things have worked very slowly,” the agent said, begging her to remain patient.

United says that it didn’t permanently lose any reservations during the transition and that every itinerary, including Shevchuk’s and Talcott’s, was eventually tracked down. The airline also says that the cutover wasn’t entirely to blame for the difficulties but that it might have exacerbated them.

A few days into the cutover, however, it was clear that things weren’t going well from a customer-service point of view.

My e-mail in-box was filling up quickly with United queries. So was Martin Hand’s. He’s United’s senior vice president for customer experience. “My e-mail volume was up by about 10,000 percent,” he told me. “For the first three weeks, I would be up until midnight every day and start again at 5 a.m., answering customers.”

It could have been much worse.

Hand says that United began developing the customer service strategy for the cutover before the airlines merged. Last October, his department started training agents in how to use Shares and briefing them on United’s new policies. It hired 400 new call center agents and recalled almost 300 furloughed employees. “We thought it was enough,” he says.

But United had bitten off a little more than it could chew. One of the critical choke points proved to be the airport agents, whose systems essentially had been downgraded from a point-and-click interface to one that accepted only text-based line commands. Although that was a temporary setback — United plans to add a more user-friendly interface by the end of the year — it combined with the recently migrated passenger reservations system and the new rules to give United and its customers a March they’d rather forget.

Asked whether he would have done anything differently in hindsight, Hand replied that “I wish we hadn’t changed so many things simultaneously.”

United passengers could have protected themselves by having a printout of their itinerary with their alphanumeric record locator number — always a good idea. That way, if their itinerary disappeared into the electronic ether, an agent would have a starting point for finding it.

And they could have packed a sense of humor. When a whole row of computers shuts down spontaneously, what is there to do except laugh? But many passengers didn’t know about the cutover, or its probable implications, until it was too late.

United says that the situation has calmed and that it’s taking steps to keep things going smoothly. Between now and June, the carrier is hiring another 400 contact center workers, adding to the staff of 6,300 employees who answer passenger inquiries via phone and e-mail. Its all-important handle times have been cut in half, to an average of 10 minutes, which is almost back to normal. (The goal is closer to eight minutes.)

No one knows when the next airline IT crisis might flare up or which airline it will hit. A combined US Airways-American Airlines, which some industry watchers are predicting, is a likely candidate. But if the madness afflicting United in March has taught passengers anything, it’s that there are some airline problems you just can’t plan for.