Southwest Airlines plans to drop its tradition of more than 50 years and start assigning seats and selling premium seating for customers who want more legroom. The airline said Thursday that it has been studying seating options and is making the changes because passenger preferences have shifted. The moves could also generate revenue and boost financial performance. Southwest made the announcement on the same day that both it and American Airlines reported a steep drop in second-quarter profit despite higher revenue. Airlines are struggling with higher costs and reduced pricing power, especially on flights within the United States, as the industry adds flights faster than the growth in travel demand. Southwest, based in Dallas, said its second-quarter profit fell 46% from a year earlier, to $367 million, as higher costs for labor, fuel and other expenses outstripped an increase in revenue.