To clarify:

on AC metal with PremEx status:

not possible with UA 500 mile certs
not possible with UA mileage
not possible with Aeroplan certs that a coworker is willing to give (unless you have AC Aeroplan status applicable to those certs)
not possible with UA SWUs, sweetspots
in other words not possible with anything other than Air Canada status and applicable AC Aeroplan certificates
OR
a friendly gate agent that feels like upgrading you (usually easiest on an oversold flight)

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