I have a UA flight, YYZ-ORD-MCO. The YYZ-ORD is a codeshare flight on AC metal. AC is about to have a labour disruption but hasn't cancelled the flight yet but it is imminent. When I called UA earlier today, they wouldn't rebook me unless I paid the difference. AC is rebooking its passengers for no cost. What can I do to have UA rebook me at this point without it costing me more?

Also, if hypothetically I can't get the rebook made, and I drive to ORD to catch the second leg, would UA ticket this to or would my flight be cancelled as I missed the first leg?