co.com - when looking at the seat map for a booked Y ticket with a waitlist F ticket (might be other circumstances as well, but that was mine):

When a seat opened on the seat map for F (but no availability for booking), the available seat turned orange. Mouse-over said to click to complete the upgrade process. Clicking got me an expected error as there were actually no seats available.

Looks like the upgrade logic on the seat map is busted and looks at open seats vs. cabin availability.