While some of the 787-10 (we have 14 left on order after various conversions) may make their living in IAD, I doubt they will be replacements for the 767-300 despite its similar range (Boeing gives it at~6400nm). It will be a huge capacity increase as it's 18 feet longer then a 787-9, so more likely a 777 replacement with somewhere around 280-290 seats.

So it depends if the company feels they can fill those extra seats out of IAD, and if they have a need for those 777's elsewhere in the system.

Of course the plan today won't be the plan tomorrow.