Since Ostrower says the 777X order is a top-up from one of the few customers for the type, if the 789 concellation is related to the 777X order, then the swap can have been decided only by one of the few airlines having both types on order. EK is not concerned (no 787 orders so far), EY and QR are obvious suspects, but do LH, ANA or CX have 789s on order ?
Like Baroque, I point my finger to Etihad. 15 789s orders were transferred from their affiliate AB to an undisclosed customer.
As far as I can see, only EY and NH are existing 777X customers with their outstanding 787 order being >15 units. NH only recently added some 787s, so that leaves EY as the most likely candidate. They have just 25 777Xs on order which I think may not be enough for their ambitions.