Finally found a map of the ICN after T2 opening:
https://www.delta.com/content/www/en...rts/seoul.html

So confirmed that transit OZ<->KE is basically taking one train to "concourse" and another to the other terminal. Presumably for security re-clearance show the connecting flight ticket, then at the actual departing terminal transit desk get the BP (that's the only potential issue, as by the time finished with train transfer it may be past check-in cut-off time for a connection time such as 90 minutes, which until Jan 18th would have been no problem if the flights are timely).