I would imagine they would pull the frame from one of the YYZ-YVR turns, it can easily be replaced by a narrow body.
The 400 seat fleet is pretty busy, I reckon they could actually use another.
Summer schedule for 2019 sees the 400 seater on:
YYZ-LHR 3x daily
That is 9x flights, out of the 12 frames available, accounting for the fact that ICN, PVG, HND, and PEK take more than 24 hours for a rotation, that is the max the fleet can sustain by my back of the napkin calculations.
In addition, the 77L fleet will be stretched pretty thin, with YYC-FRA daily, YYZ-FRA daily (likely to see frames doing YYZ-FRA-YYC-FRA-YYZ rotations), YYZ-HKG daily and YYZ-YVR-SYD daily.
Not much slack in the 777 fleet at all.
Pulling one frame from the YVR-YYZ turns would make perfect sense, indeed. And the 77L is streched pretty thin!
Regarding the 12 400-seat 777-300ERs, I believe YYZ-PVG will be operated with the 789 and that YVR-PVG will use the 77W, but that doesn't change much in terms of usage. 9x flights with 12 frames is pretty much the maximum it can sustain.
On another note, it's interesting to see that the 788 will not even transit through YUL at all next summer.
YYZ-PVG will be 77W, website reflects. YVR-PVG also shows as a 77W, I suspect YYZ will see the 400 seater and YVR the 450.
I imagine all 788 flying will be out of YVR next summer, YYZ hasn’t seen them in ages, and YUL is upguaging.
For July-August, the AC website is actually showing 789 for YYZ-PVG (AC027/AC028).
As for the 788, it will operate the following flights:
(Daily) AC 868 YYZ-LHR 0910-2100 788
(Daily) AC 859 LHR-YYZ 1800-2040 788
(Daily) AC 896 YVR-LHR 2105-1420+ 788
(Daily) AC 855 LHR-YVR 1005-1130 788
24567 AC 802 YVR-ZRH 1300-0755+ 788
13567 AC 803 ZRH-YVR 1005-1125 788
1357 AC 806 YVR-CDG 1245-0735+ 788
1246 AC 807 CDG-YVR 0925-1020 788
12346 AC 039 YVR-KIX 1330-1545+ 788
23457 AC 040 KIX-YVR 1715-1010 788
(Daily) AC 035 YVR-BNE 2350-0725++ 788
(Daily) AC 036 BNE-YVR 1040-0710 788
(Daily) AC 894 YYZ-MXP 2040-1030 788
(Daily) AC 895 MXP-YYZ 1210-1500 788
So they will see them in YYZ next summer for a change.
Next flights: YQM-YUL-MIA-YUL-YQM / YQM-YUL-YQM / YQM-YUL-LYS-YUL-YQM / YQM-YUL-FRA-MLA-FRA-YUL-YQM