It's actually pretty easy for (e.g.) United to add or remove rows of their Economy Plus seating. They can make that change overnight. When United and Continen

It's actually pretty easy for (e.g.) United to add or remove rows of their Economy Plus seating. They can make that change overnight. When United and Continental merged and began adding E+ to the Continental fleet, most aircraft were completed pretty quickly since that change can be made during overnight maintenance at the hubs.