A few other thoughts as to why I don't think this is possible (and is it with any airline?)

When purchasing a revenue ticket from EK, they act as the travel agent (and in fact are registered as a TA with IATA), typically another TA cannot just touch, modify or amend a booking not made by them. When wanting to upgrade with Skypass miles KE as your TA would have to make a change to the booking, which they wouldn't be able to.

Effectively KE will pay EK cash for the upgrade, and you pay KE with miles, so it's relatively straight forward ticket change, but I don't think such a mechanism to process an upgrade in this way exists. Even if wanting to upgrade a Y Award to a J Award my understanding is that it is cancelled and a new one issued, rather than amended.

Lastly if no redemption chart is published this would be a give away. Being unfamiliar with Skypass I'm not sure how easy redemption charts are to access in general so this might not be helpful.

Ultimately this is a 50:50 whether this question is best answered here in the EK forum or rather by the Skypass forum, as it ultimately revolves around Skypass redemption.

EDIT: Just realised OP had written KA, should be KE = Korean Air. KA = Cathay Dragon