Originally Posted by findark

Until it clears, no one will notice or care.

If it clears, either you'll get to sit in J or they'll notice the error at that point (or at check-in) and kick you back to Y. Waitlist clearance often triggers a manual revalidation of the ticket, but in the case of an upgrade waitlist I'm not sure if that's required so I don't know the odds here. The computer doesn't know that PZ is wrong, so it's a question of whether a human is caused to look at it and notice.