With a water-cannon salute, balloons and commemorative bag tags, Southwest Airlines on Tuesday officially retired its decades-old practice of open seating. For the first time, passengers received assigned seats, a major shift for the carrier, which also introduced a new group boarding process and premium seat options.

The first two flights with assigned seating landed early in the day, at Chicago Midway International Airport and then at Orlando International Airport, where a plane arriving from San Juan, P.R., glided toward the jetway under two arcs of water, a salute reserved for special occasions.

The celebration continued at the gate, which was festooned with red, blue and orange balloons. Passengers loaded up on free snacks and coffee and took selfies through heart-shaped holes in giant boarding passes. The airline handed out bag tags with the line, “I understood the assignment.”

Southwest says the seating changes are intended to keep up with evolving customer expectations and to increase revenue.

Tony Roach, an executive vice president at the airline who was on the flight from San Juan, said that the airline had “outgrown” open seating, adding that assigned seating also reduces anxiety among travelers because they know what to expect.

“There were a lot of limitations with open seating in terms of what we want to do in the future,” he said. “You aren’t able to offer things inside the cabin which a lot of people want. Things like extra legroom would have been very difficult to do with an open-seating environment.”

For more than 50 years, Southwest used a system of open seating: Once they had checked in, passengers received a group and a number that determined the general order of when they’d get on the plane. Then once on board, they could choose any open seat. Getting a desired seat, or sitting together, often hinged on checking in early, and there were no premium seats.

The system was loved by some travelers, who viewed it as egalitarian and flexible, and bemoaned by others who complained that it was chaotic and stressful.

Now, Southwest offers three categories of seats — standard, preferred and extra legroom — and the most expensive can cost hundreds of dollars more than the cheapest fare. Boarding order now depends on the fare class, as well as whether a traveler has elite status with the airline or holds its credit card. Travelers can also pay extra to board in an earlier group.

As of Tuesday, the airline also ended its flexible policy for plus-size passengers, which allowed customers to ask for an extra seat at the gate, or pay for an extra seat and later receive a refund. Those passengers will now need to purchase two seats ahead of time, without a guaranteed refund.

Some loyal travelers expressed dismay that the airline was abandoning some of the quirks that made it different from other major U.S. carriers. In May, Southwest eliminated its popular policy of granting travelers two free checked bags.

Larry Wolf, 74, who has flown with Southwest for more than two decades, said he was enticed early on by the airline’s reputation as an industry disrupter.

“Iliked them really well when they were young and scrappy. In the past, they seemed to have mastered the quick turnaround, they were able to get back on time after delays, they are very agile with swapping planes and all of that,” said Mr. Wolf, who was at LaGuardia Airport awaiting his flight home to Chicago.

With assigned seats and premium fares, “now it’s just like any other airline,” Mr. Wolf said.

Several longtime Southwest fliers also remarked that they enjoyed the unpredictability of the previous open-seating system. Aaron Schottenstein, a 35-year-old who lives in Phoenix, said he’d been dreading the changes.

“I loved the boarding, getting on in the lines, yapping with everybody while you wait there, talking about how people are trying to get like five spots closer because they might get that much better of a seat,” said Mr. Schottenstein, who was traveling from New York to Phoenix, with a connection in Kansas City.

Others welcomed assigned seating, saying it eliminated pain points like checking in 24 hours ahead of a flight to improve the chances of snagging a better seat.

“I love it,” said Maureen Hager, of Buffalo, who was traveling home from Orlando on Tuesday with Veronica, her adult daughter. “I hated waiting on line, trying to get there early. I don’t like waiting to get to the airport and waiting on line and the anxiety of not knowing if you got seats together.”

Veronica, however, disagreed. She called the change a “money grab.”

“I like the other way,” she said. “I like the thrill of having to check in, trying to get Group A, first round.”

Bob Jordan, the airline’s chief executive said in an interview last month that the airline was confident that customers, who he said desire these changes, would stay loyal.

“We’re now offering things that the vast majority of people want,” he said, adding, “We’ve seen no defection from the brand.”

Eric Adelson contributed reporting from Orlando, Fla.

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