Do I understand that your transatlantic flights are on AC flight numbers and are confirmed, and that the issue here is the two flights to/from San Francisco? Also, I'm guessing that you chose the UA metal to connect with AC's 777 at YYZ because there are alternate flight combinations on AC metal to LHR, via YYZ (an hour later), or YUL, but no 777. If I'm wrong, and the 777 isn't the reason you booked this particular combination of outbound flights, then AC has a lot of ways to protect you on its own metal.