Correct!

This has been my experience as well. Last year I was booked SIN > LAX > SFO > YYZ with UA handling the first two segments and AC handling the last segment. As we all know UA cancelled SIN > LAX service at the end of the year and I had to call UA for rebooking options. A 10 minute phone call later and UA rebooked me on the SIN > SFO service then changed my AC flight to match the new departure time. The point being as the UA agents can and do have the power to touch segments operated by other * partners when the ticket is issued by UA. At the end of the day your contract is with UA not the partners. The partners are merely airlines that fill sectors in the flights.

Safe Travels,

James