No.
A retcon is a form of narrative tool that change a data of the story to shade a new light on pre-existing elements.
This new light can indeed erase the pre-existing narrative data completely or enhance it. In the case of One Piece, Nika is indeed technically a retcon. But not one that erease the data of the story. Rather the opposite. The reveal recontextualizes elements already present but hidden in plain sight (the mythogony, the powers of the fruits of Luffy, the thematic of the story, the contextual attribution of the fruit, the antagonism in response etc.)
Nika is what we call a positive retcon.
A retcon is a form of narrative tool that change a data of the story to shade a new light on pre-existing elements.
This new light can indeed erase the pre-existing narrative data completely or enhance it. In the case of One Piece, Nika is indeed technically a retcon. But not one that erease the data of the story. Rather the opposite. The reveal recontextualizes elements already present but hidden in plain sight (the mythogony, the powers of the fruits of Luffy, the thematic of the story, the contextual attribution of the fruit, the antagonism in response etc.)
Nika is what we call a positive retcon.
"A retcon is a form of narrative tool that change a data of the story..."
You are describing plot progression, dude.

