Originally Posted by
flyerCO
When you used a GUC they flights change from DL coded to operating airline code.
Technically you only are supposed to earn based on what you paid. However if the fare class is an earning one you end up due to system issue earning based on the upgraded flight. Since Z doesn't earn you'll end up needing to request credit manually. Thus likely will earn as you technically should based on the un-upgraded flights.
Similar issue with VS flight upgraded via GUC - clears into G class which earns nothing (award fare code). Required quite a bit of back and forth with the SkyMiles folks but eventually got there.