Originally Posted by Stranger

If the entire itinerary is in Y, sure. But even then, price might have changed. If that leg is, for instance, part of an itinerary in P, for which there was no P available at the time of reservation, rhen it's a different story.

I recently went through a similar exercise. Having an intra-Europe leg in Y on an otherwise P itinerary. After booking that leg under the AC codeshare number on aircanada.com, I called them and asked them to switch to the actual flight number, which they did, entailing reissue but same booking reference. At the time there was no SA upgrade inventory available however. Still it recently opened up and we did upgrade.

This said, I believe they were only able to reissue because it was within the 24 hours window.