I'll add my own data point here. I called SQ first. I had my pick of the 3 daily LAX options. I went with SQ35 non-stop. They hold the reservation for 3 days.
Then i just had to call Aeroplan for them to reissue the new ticket. Aeroplan also offered me an economy United flight to LAX but i declined since it would have involved a 12 hr layover at LAX.
I'll just get to LA on my own dime a week earlier to spend with family