Budget airlines are hoping a big shift in how they assign seating will win over a broader passenger base, in a desperate bid to give their shares a much-needed lift. Low-fare carriers have struggled to keep pace with legacy rivals’ aggressive discounting, and their costs have crept up after agreeing to hefty pay increases for pilots and cabin crew members. Like Southwest, both Frontier Group and Spirit Airlines are planning to allow customers to pay for more space and other perks.