It’s happening: Southwest Airlines will start charging passengers to check bags for the first time.

It’s a stunning reversal that shows the low-cost pioneer is willing to part with a customer perk executives have said set it apart from rivals in more than half a century of flying in hopes of increasing revenue.

Southwest’s changes come after months of pressure from activist Elliott Investment Management. The firm took a stake in the airline last year and won five board seats as it pushed for quick changes at the company, which held on for decades — until now — to perks such as free checked bags, changeable tickets and open seating.

For tickets purchased on or after May 28, Southwest customers in all but the top tier-fare class will have to pay to check bags, though there will be exceptions. Elite frequent flyers who hold “A-List Preferred” status will still get two bags and A-List level members will get one free checked bag. Southwest credit card holders will also get one free checked bag.