2 things to keep in mind:

As a Prestige, you can only upgrade Tango Plus M fares (48 hours in advance) and Latitude Y and B fares (at time of booking)

(The summer upgrade promo announced today that allows *P to upgrade any Tango Plus fare only applies to domestic and US travel...therefore if not on an M fare or Latitude fare, you will only be able to upgrade the YYC-YVR portion, not the YVR-HKG portion)

If the flight is booked via Lufthansa, it depends which flight # it is booked under. If the flight is booked as a codeshare with a LH flight number (i.e. LH4567 operated by Air Canada), then you will not be able to request an upgrade in advance, and will only be able to request an upgrade at check in. And even then, the booking class of the LH flight might not map to the same on AC and therefore you might not be able to upgrade anyways if the booking class is ineligible.

If the flight is booked as an AC flight # (i.e AC007) but issued on LH ticket stock, you should still be able to request an upgrade in advance.