Southwest Airlines Co. will begin offering assigned seats, ditching the free-for-all policy that has been a defining feature of the carrier for more than half a century. The airline announced the seismic change to its business model Thursday alongside a new premium-class option and plans for redeye flights. While Southwest indicated earlier this year that it was reconsidering the seating policy, it’s now facing heightened pressure to revamp underperforming operations from activist investor...