Both airlines we've got up here load "last year's" schedule initially, 11 months in advance of travel. Then they make adjustments on two dates each year. And the non-appearance of 787's on the existing (read: 2018) winter schedule would suggest this policy hasn't changed.
My prediction?
787s:
0.5 or 1 for YYC-LGW
1 or 1.5 for YYC-various Hawaii flights to replace the 767 that's dedicated to those routes
1 stays on the current YYC-YYZ daily round-trip
Of course, that frees up a 767. Which I think would be dedicated 0.5 to Toronto-Hawaii. (While the remaining '67s flights continue: 1 for YYZ-LGW, 0.5 for YEG-Hawaii + spare)