The terms are in the GUC. Valid on Delta-marketed, Korean Air-operated flights for use globally on Delta's published fares booked in J class to Korean Air's A class, Y or B class to Korean Air's Z class, or booked in M class to Korean Air's O class.

These tend to be more expensive upgrade paths, almost like buying PS to get D1. I have been able to get Delta to book me into M when I found O available on KE but I believe that is an award space so its not always available. I have not tried to do the other fares but they would probably be even more expensive and it may not be worth it for a shorter connection.

Also, I hope you got your SLC-MSP upgraded to first as part of your GUC use.