Southwest Airlines, the second-largest carrier in Atlanta, announced plans to end open seating and move to assigned seats.

The Dallas-based airline signaled earlier this year that it was looking at making such a change and made an announcement Thursday making it official and saying it expects the changes to take effect with bookings starting next year.

Southwest has allowed passengers to board and pick their own seats since it started operations in the 1970s. But the carrier said it made the decision to assign seats and offer premium seats on all flights after conducting “extensive research” and listening to customers.

“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,” said Southwest CEO Bob Jordan in a written statement.

The move to assigned seats also enables premium seating, which will be a portion of the cabin with extended legroom, according to Southwest. It’s expected to make up about a third of seats across its fleet.

That change is part of an effort by Southwest to improve its financial performance, with Jordan saying it “will unlock new sources of revenue.”

It’s also a return to the past of another sort for Southwest. The airline entered the Atlanta market through the acquisition of AirTran Airways, which had an Atlanta hub and offered seat assignments and business class seats. Southwest absorbed AirTran into its operations and converted it to Southwest’s open seating model.

While Atlanta-based Delta Air Lines and other carriers across the industry have seat assignments, Southwest has been known for having open seating for more than 50 years.

Some Southwest loyalists prefer the system of choosing their own seats when boarding and some of the flexibility it brings — especially when they get priority boarding.

But according to Southwest, “preferences have evolved with more customers taking longer flights where a seat assignment is preferred” and it found 80% of Southwest customers and 86% of potential customers prefer an assigned seat. Open seating can be a concern for some, including families that need to sit together and connecting passengers who have to rush to the next gate to make a flight and end up as the last to board.

“When a customer elects to stop flying with Southwest and chooses a competitor, open seating is cited as the number one reason for the change,” Southwest said. “By moving to an assigned seating model, Southwest expects to broaden its appeal and attract more flying from its current and future customers.”

Joe Leader, CEO of the Airline Passenger Experience Association, called Southwest’s boarding system “the most elegant boarding solution for passengers that arrive to the gate on time.”

Southwest has “a super fan base that loves Southwest exactly how it is,” Leader said. “Now, as Southwest looks to assign seating, they have to thread a really careful needle.”

Zachary Warring, an analyst from CFRA Research, said in a written statement that the end of the open seating policy is “a move to turn around its margin problem, but we believe may upset its customer base.”