I had done that before but more originating from YOW or YYZ where I changed an UA operated ticket booked on AC to AC operated flights with the same day change fee due to schedule changes.

With an ticket out of LAX operated by UA, you would need to check in at Term 7 rather than Term 2 so it may be a little difficult.. But if you can get to the airport earlier in the day and try AC at Term 2, maybe you can get lucky and always have the fall back plan to do the UA flight if it is refused.