Does GNI change once it has been published? From what I understand, GNI is published for the last year once the year has ended  

I am trying to have a look at the GNI for Korea for the past couple of years  

The official GNI is published by Bank of Korea on their website  

10 days ago I checked the GNI, and the latest published GNI was for 2016, and it was 3198.40 * 10000 Won (about $30000)  

Today I checked the GNI again and the system shows the GNI for 2016 and 2017 as:  

2016: 3212.40 * 10000 Won (higher than previous value of 3198.40)
2017: 3363.60 * 10000 Won  

Why is the GNI for 2016 different after 10 days? How does this work? Does this mean the GNI for 2017 as published now is not final either? Can either of these GNI values change again?
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