Yes, it has to by Y/B/M on Delta metal and have Z inventory available to upgrade. You can route on DTW-ICN for 'direct' or through NRT from various departure points like JFK/MSP/SEA etc.

The one thing to be careful with regarding the skyteam upgrade is that even if you book a full Y ticket on KE and call Delta to use miles, there still has to be upgrade space available from KE for it to be processed, which is out of Delta's control... they just check if there is space and will upgrade you if possible.

And yes for a skyteam upgrade you need to be ticketed on the operating carrier... if you book a DL codeshare you might not be able to upgrade on KE.

I'd suggest the advice given earlier though... a discount biz fare on KE may be cheaper than the full Y fare. Same with Delta as well.