• Southwest plans to offer pricier seats with extra legroom and end open seating on its planes.
• The shifts are the most major in Southwest's more than five decades of flying.
• Southwest expects to start selling seats with the new cabin option next year.

Southwest Airlines is ending open seating and will offer extra legroom seats on its airplanes as mounting pressure on the carrier to increase revenue prompts the biggest changes to its business model in its 53 years of flying.

The airline plans to start selling the first flights that will offer extra legroom next year, it said Thursday. The airline also plans to start overnight flights, starting in February.

Southwest executives have said for years that they were studying such changes and hinted in April that it was seriously considering assigning seats and offering pricier seats with more legroom. The airline currently puts customers in one of three boarding groups and assigns a number, setting off a mad dash to check in a day before the flight. Customers can get earlier boarding though if they pay for a higher-priced ticket, they'll get a better boarding slot.

When travelers choose a competitor over Southwest, the airline found in its research that its open seating model was the No. 1 reason for that choice, the carrier said in a release that outlined the changes. It also said 80% of its own customers prefer an assigned seat.

"Although our unique open seating model has been a part of Southwest Airlines since our inception, our thoughtful and extensive research makes it clear this is the right choice — at the right time — for our Customers, our People, and our Shareholders," CEO Bob Jordan said in a news release Thursday.

The airline is under even more pressure now to segment its product like other airlines after activist investor Elliott Investment Management disclosed in June a nearly $2 billion stake in the airline and called for new leadership as the airline underperformed competitors.

"We will adapt as our customers' needs adapt," Jordan said at an industry event last month.

Southwest said it expects about a third of the seats on its Boeing 737s will offer "extended legroom, in line with that offered by industry peers on narrowbody aircraft." The Federal Aviation Administration would need to approve the cabin layouts, the airline added.

The Dallas-based airline had prided itself and raked in steady profits for most of its more than five decades of flying on its simple business model. Jordan said last month that not assigning seats was easier to offer when planes weren't so full.

Analysts criticized Southwest for moving too slowly. Rival carriers offer a host of options to upsell customers like extra legroom seats, premium economy or business class. Other airlines, however, like Delta, United and American, four years ago took a cue from Southwest and ended flight-change fees for most tickets.

Southwest will provide more details about the upcoming changes at an investor day at the end of September.