That did not go well. The new reservation with the Air Canada flight apparently showed the old eticket number with the United flight. When it came time to check in with Air Canada, the check-in agent could only see the booking ticketed with the United flight (for a different day). Calling United did not help at all, other than reading the Air Canada PNR (which was already used at check in) and the old United eticket number back to me.

Luckily, a United agent at the nearby United check-in counter was able to find the itinerary with a new eticket number that was never shown on united.com or aircanada.com. The Air Canada agent was able to complete the check-in with a minute to spare.

How could this be avoided in the future? Does Air Canada have the ability to look up the new eticket number that should have been attached to the reservation?