Hi, I'm new to the forum and would like some help regarding a muddy codeshare situation I find myself in.

I booked the Air Canada operated AC781 YUL-LAX on Dec 31 from UA, marketed as UA8467. A few days ago, I received an email showing a "schedule change" that changed the arrival time of the flight by like 20 minutes. We also got new 13digit ticket numbers. I logged onto the air Canada trip portal where there was a prompt to confirm the schedule change, with a warning that "
once you confirm the schedule change, you might not see the ticket in the original booking source". I confirmed it. After confirming it, I went back to the United portal, which shows that "One of the flights in your itinerary has been canceled or is still not confirmed." And this is when the chaos begins.

I contacted AC customer support. They told me first that "The flight had been cancelled by United Airlines you will need to contact United Air for rebooking assistance." I asked more, and they then replied "the ticket is still valid, but "make sure that everything is properly confirmed with United Airlines, due to the fact that we only had a slight schedule change for which you were notified about". Another employee confirmed on the phone that the ticket is still valid.

When I contacted united, they seem to be on a different page. This is the answer I got :" The non-stop flight is still operational, however, Air Canada didn't confirm the space on their end resulting the flight to be cancelled on our system. At the moment, what we can do best is to rebook the trip to a United operated listed below." The UA people I talked with on the phone gave similar answers.

Interestingly, the AC flight number when I booked it was AC781, and now it changed to AC783. I am puzzled as to what's going on . My best guess is AC won't give out the codeshare spots to UA after they change their flight schedule (and possibly operating plane), so everyone who booked with UA codeshares got their ticket annulled? Or there's a technical error that prevents UA from validating already ticketed tickets?

If I don't ask for a UA rebook (since I prefer the direct flight), should I be able to still get on the flight on Dec 31st?