Originally Posted by
Pigeon49
"
I believe the oldest 77Es at LHR are being replaced 1-for-1 with the 777-9s."
Ever the optimist you are to assume the 777X will ever go into production! 
well technically it’s already in production. They’ve built over 20 already and a Singapore 777-9 rolled out of the factory last night
I assume certification. Well yes it will as it’s about to receive approval for final certification flights from the FAA.