Hello there, just wanted to make sure I understand use of GUC on Korean Airlines. I've always just YOLO attempted it before and either its worked or not worked when I called and I didn't really check the behind the scenes reasons why it didn't work to get to Prestige.

I have a RT from LAX to ICN coming up this year (well, going past Seoul but I'll have to switch airlines after that point). When I did this same routing earlier this year I did LAX-ICN outbound (but wasn't able to upgrade) and then ICN-SEA-LAX on the way for easier upgrade to PS and then to D1.

Work generally buys a Y class ticket using a Delta flight number (and the nonstop from LAX to ICN is only on Korean metal). Is the whole thing just making sure (using ExpertFlyer) that Korean's flight number for that code share has Z class seats available? I've looked around when I'd be traveling and some days have 0 Z availability and some have a few. If I book the nonstop I want to make sure that I can upgrade, otherwise I'll do the less convenient LAX-SEA-ICN since SEA-ICN is on Delta metal so more transparent as to whether I can use a GUC.

I prefer Delta One to Prestige, but at least on the outbound leg the schedule of going LAX-ICN directly is better for my onward flights than if I did LAX-SEA-ICN.

tl;dr - Do I just have to make sure my that my ticket is Y for the delta flight number, and then that Z is available for the same flight using the Korean Airlines flight number for the same flight?