Yes, the identity of the KON keeps changing because the empires that stepped in the role stopped existing altogether.
For e.g. t Nazi-Fascist Germany became the KON in 1919, because the Central Powers dissolved in 1918. Similarly, from 1945, the Soviet became the KON, because Nazi Germany was dissolved in 1944-45. Soviet ceased as the KON in 1991 because it disintegrated. I don't think there has been a time when the WT changed the identity of the KON while the empire was still in existence.
What we see here is a succession of empires that have stepped in the role of the KON.
"King of the North" is a title, same as what we call "The President of the USA" or "Pope". The title remains the same, but many individuals will fill in the roles in succession.
For e.g., when we say "The President of the USA had declared war on Nazi Germany", "The President of the USA had resigned due to the watergate scandal", "The President of the USA decided to build a wall on the border", we are here speaking about one title, but different individuals. The individual changes, the title remains.
I don't think WT has flip-flopped regarding the identity of the KON. Flip-flop means reversal, and I don't think WT has ever reversed itself here. , i.e. they told that USSR was the KON, but then told that it isn't- while USSR was still in existence.