Does GNI change once it has been published?
GNI is GDP plus net primary income from abroad

Any such numbers are subject to revision as more information is obtained; in many countries such numbers are typically revised every three months as part of a National Accounts and Balance of Payments update, with more substantial revisions annually.  Korea's latest update was published on 28 March

The fact that you did not see 2017 data the first time and did see 2017 data the second time was a pointer to these two readings being across such a revision point   

Such numbers are typically never final, and potentially keep on changing, though after long periods revisions to numbers then tend to be affected more by methodology changes than by the arrival of new information