Originally Posted by
transportprof
It's not possible to upgrade a UA plated ticket to premium economy on AC, except perhaps on the day of travel at the airport.
If you're still within the 24 hour ticket purchase window, my recommendation is to cancel with United and rebook with Air Canada.
You can't "upgrade" the ticket but you can absolutely fly AC Premium Economy on a United ticket. You need to "change" the ticket to PE - if within 24 hours cancel and rebook is easier. This is something that would need to be done by calling United, and it would help to know which booking codes are needed (B on the short segments and probably E or N for the long ones).
If it's outside of the 24-hour window however you might not be able to avoid paying a change fee. Also note that it requires switching to the native AC flight number.