I had a similiar situation in October. I was booked SEA-LAX-(NRT)-SIN (but in coach), not upgraded. My original SEA-LAX flight was scheduled for 9am. A couple days before my flight I noticed the SEA-LAX was zeroed out, so that they were likely looking for volunteers. I asked at check-in and they said they weren't looking for any, and that it appeared ok.

Went to the RCC and asked to be put on the VDB list just in case. Went up to the gate at 8:20, and it was a mad house - enormous line of folks waiting to talk to gate agents. Smiled, knowing that the inevitable announcement would be coming soon.

They announced that they were looking for volunteers, and I was the first there. Explained that I was willing to be bumped, and that they could re-route me on the SEA-NRT direct flight that was wide open, and then I could connect in NRT to my original NRT-SIN leg. They were very glad to get a volunteer, as few people were biting at the offer, and were glad to know that I had already checked out information on the re-route for them.

My wife and I got $600 in travel vouchers each for the bump.

Mileage Plus only credited me for the SEA-NRT-SIN. I called them up and had them change it to my original ticketed itinerary of SEA-LAX-(NRT)-SIN because it was more miles, and I had paid slightly more to be routed that way. You might have a harder problem getting Air Canada to correct this for you since they probably don't have as good of visibility in to United's ticketing and reservation systems to see what happened.

I don't think you would get bumped to the SEA-SFO flight, that leaves only 10 minutes later. I don't think they would have enough time after getting the SEA-LAX flight out of the way (which is usually there first priority) to get you rebooked in time to catch the SEA-SFO. In addition to the SEA-NRT direct flight, you might possibly try getting re-routed as:

UX 6862 SEA-PDX lv 8:15 ar 9:05
UA 746 PDX-SFO lv 10:00 ar 11:40
UA 853 SFO-NRT lv 12:40 ar 17:05
UA 897 NRT-SIN lv 17:40 ar 23:55

But that might set up an illegal connection in NRT, I'm not sure.