Points Transfer to United/Air Canada

I booked a flight from Denver to London through United and noticed that it’s also being marketed by/offered through Air Canada. I would love to use my Amex points to upgrade my seat to business class. I know I’m able to transfer points to AC via Star Alliance but since I booked the flight through United, will I be able to use those AC/Star Alliance points to upgrade? Or would I need to cancel my booking through United and rebook the same flight through Air Canada?

Appreciate any guidance!