Retcon is shorthand for Retroactive Continuity, and it was popularized by comic book writers (which led to many, many, many people misusing the term).
In order for something to be a retcon it has to violate a story's narrative or world building continuity in some way.
Examples of Retcon within FFXI:
Lady Lilith's Vana'diel was the true outcome of the Crystal War, and our Vana'diel is just Altana's desired outcome that she forces to happen. We were never just heroic adventurers, but Goddess Chosen with Divine Dispensation.
We didn't actually kill The Shadowlord. The Zilart Princes and Ark Angels did (in spite of our later revealed Divine Dispensation).
Examples of Not-Retcons, merely additions within FFXI:
Within the first 5000 years of The Meltdown, a draconic entity threatened the whole world. Odin and a Heroic Galka took him out behind the shed, and put him down. He entered stasis within an egg or sixteen, because he's a weird quasi-dimensional dragon that drinks tesseract Crystal juice(Creation energy).
The Olduum Civilization was all about lightning, and may have created Maroliths, and passed the caretaking of their special control staff into the hands of the Mamool Ja.
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Use of the word can get a bit murky, because a lot of things can affect continuity. Even things added that were intended to have no bearing on the story. XI has plenty of retcons within its own framework, some of them are pretty hard asks if you really care, but there really aren't a lot within TVR. They did good with what they had to work with, as always.