Southwest Airlines will host a blockbuster event in Dallas this week, to unveil a service most other carriers began offering decades ago. At its annual Investor Day, Southwest will showcase its first-ever plan for assigning passengers to specific airplane seats, including premium-priced larger ones that will require altering the layouts of the company’s previously egalitarian cabins.Selling a variety of seating options at different price points has long been the practice at Fort Worth–based American Airlines—and at Delta, and United, and most other major carriers around the world. Yet some insiders think Southwest will treat the announcement with the sort of fanfare that accompanied Apple’s introduction of the first iPhone—framing it as a this-changes-everything moment for the company.Even if Southwest CEO Bob Jordan doesn’t don Steve…