Boeing Co. is wrestling with one of the thorniest problems in business: How and when do you replace a successful product?

The aircraft giant’s big 777 twin jet needs a makeover, according to the airlines and lessors who have made the plane a mainstay on long-haul routes since its introduction in 1995.

The 777 pays Boeing’s bills, delivering as much as $1.2 billion in revenue each month, based on its average sale price for the largest and most popular model, according to aviation consulting firm Avitas.

Tags: boeing