DALLAS >> Southwest Airlines announced a three-year plan on Thursday to revitalize its operation and customer offerings as it defends against an activist investor calling for new leadership and a strategy overhaul.

The plan expands on a series of changes by Southwest in recent months, including plans to add premium seats, introduce red-eye flights and replace its pick-your-own seating system with assigned seats, starting in 2026.

The airline said on Thursday that it would begin selling vacation packages and was partnering with international airlines, starting with a connection in Baltimore via Icelandair next year. It also announced a $2.5 billion share repurchase program and plans for operational changes, including speeding up how long it takes to get planes back in the air after they have landed and finding other ways to reduce costs.

Southwest shares were up about 6% Thursday afternoon on the announcement.

Bob Jordan, Southwest’s CEO, presented the initiative during a meeting with investors and analysts on Thursday morning, calling it “the most transformational plan” in the airline’s history.

“We’ve laid out a plan that is built on the values and the principles that generated years of financial success and shareholder returns that garnered admiration, loyalty and love,” he said.

The airline is making the moves under rising pressure from an activist hedge fund, Elliott Management, which has amassed a stake of more than 10%, worth almost $2 billion. Elliott has said the airline is underperforming and has placed blame on Jordan, who has worked at the airline for decades.

The hedge fund contends that Southwest can better contain rising costs and improve profit margins. To do that, Elliott says, the airline needs new board members and a new chief executive. The hedge fund plans to call for a shareholder meeting as soon as next week to vote on candidates it has proposed for the airline’s board, mostly former industry executives.

Jordan on Thursday said that Elliott has refused to work with the airline, focusing instead on “tactics and on gamesmanship.” The airline also said that it had appointed a new board member, Robert Fornaro, the former chief executive of both Spirit Airlines and AirTran, which Southwest acquired more than a decade ago.

Elliott did not immediately respond to a request for comment about Southwest’s Thursday announcement.

Southwest once reported strong profits consistently even as other airlines lost money and sought bankruptcy protection. But its financial results in recent years have disappointed some investors, who point to airlines that have adapted more quickly to shifting demand by, for example, selling more premium seats.

Jordan has acknowledged that the airline needs to make some changes. The airline is in the middle of rolling out better internet service, in-seat power outlets and larger overhead bins. This year, the airline also began listing its flights on travel websites like Google Flights and Kayak.

But it is not clear whether Southwest, which has become one of the world’s biggest airlines by being different, can now do well by acting a little more like other airlines.

“They don’t need to become another legacy airline,” said Savanthi Syth, a Raymond James Financial analyst. “They just need to adapt, and they need to differentiate themselves.”

In a note about the plan the company put out on Thursday, Syth said it was promising, but that “execution will be key.” Analysts with the investment firm Jefferies also commended Southwest for taking quick action, but said in a note that some of the airline’s projections seemed optimistic.

Other carriers have spent the past two decades redesigning their planes, fares and policies. Unlike Southwest, many airlines now charge bag fees and offer restrictive basic economy fares to bargain hunters and premium seats to more affluent travelers. Those changes are widely credited with helping airlines make more money.

Southwest made changes, too, such as selling priority boarding, but it sat out much of that industry evolution. Southwest lost a competitive edge in recent years, after other big airlines dropped change fees for many travelers.

Now it’s trying to catch up.

Southwest provided more detail on Thursday about the changes it announced over the summer, which were informed by surveys, interviews and focus groups with customers over the past year.

The airline said that four of every five customers wanted assigned seats, which it will start selling next year for flights in 2026. It has used open seating for decades, allowing it to quickly load planes, but customers have grown frustrated by the experience, Southwest said. Despite switching to assigned seats, the airline will still ask customers to line up by number to board, which it says will save time.

Southwest also said it would start offering red-eye flights in February, adding the equivalent of 18 new planes of capacity over next year. And the airline defended offering two free checked bags to each passenger, saying that a review found that changing that policy would do more harm than good.