what you see is what you get on the United sfo/lax to hawaii. You can't standby same day for business, etc. However, *IF* you book the mixed award you can keep looking to see if UA opens any aeroplan business seats. If so it'd be free to call Aeroplan and change that segment. Realistically you have a better chance of driving a Tesla over a bridge from sfo or lax to hawaii than of united opening business saver seats.

One advantage of starting yyz-sfo/lax in J is that you might be able to pick a widebody flight with lie flat pods. Another advantage is your baggage allowance is determined by the first segment. If this was reversed I think you'd be paying UA first/second checked bag fees.