I think my go-to example for a time we got an obvious retcon* whereby the change was well-thought-out and made the world feel alive is the reinterpretation of the elemental wheel in Shadowbringers.
* Using the word "retcon" literally here, retroactive continuity. There's a sub-type we all hate - the "subtractive retcon" - where they basically say, "There's a contradiction, so pretend that other thing never happened." The XIV devs seem to take it as a personal challenge to avoid them in favor of "additive retcons" wherever possible (the ones that frame the change as a new revelation; the old lore still happened but is now considered an in-world misunderstanding / mistaken assumption). From a "Watsonian" perspective, addition retcons are easier to see as part of the story itself, or even to mistake for planned misdirection or foreshadowing.Note that, in such cases, nothing relies on the fans just making stuff up to make sense of it. It's not one of the cases where there's no citation and people bypass "theory" in favor of "it makes sense to me so it's as good as canon". The writers themselves came up with, explained, and wrote stories consistent with a change to what was previously established in such a way that it feels almost like it was true the whole time.
- In 1.x, Light and Darkness weren't part of the game; we only had the elemental polarities astral and umbral, which were clearly themed after "brightness/life" and "shadow/death".
- In 1.x, it was said that the polarities were an elemental phenomenon.
- In 1.x, it was said that calamities were of the elements and it was expected that there would be six, but it turns out there would be a seventh (implying the potential for at least an 8th if it turned out the fact they went in order, one each, was not a coincidence).
- In ARR, it was said that the Seventh Umbral Era (Bahamut) was all the elements at once with an astral charge.
- In ARR, it was said that the introduction of Light and Darkness was a new type of energy; Light and Darkness were not of the elements.
- If we were NOT going to break pattern, the Eighth Umbral Calamity should itself be all the elements at once with an umbral charge.
- Where the story was at the time, the creative team wanted a "Light Calamity" story.
- Remember: Calamities are elemental, astral and umbral are of the elements, Light and Darkness are not, and astral has already been used.
- Shadowbringers then comes in and says, ACTUALLYYY, Eorzean aetherology has it all backwards. Astral and umbral charges come from the influence of Light and Darkness, but LIGHT causes we call the shadow one and DARKNESS causes what we call the bright one, thus making it possible for an umbral-aspected calamity to be Light-themed.
- In doing so, they also made it so that "umbral" (which was always themed as "low" or "nether" or "underground") ended up being "closer to Hydaelyn" (making that more thematically important than its association with shadow), while "astral" (which was always themed as "high" or "heavens" or "sky") ended up being "closer to Zodiark" (making that more thematically important than its association with "brightness"). It also set up the "Ice is the closest element to Light" passivity-association angle that rhymed well thematically with other story beats like Shiva and all that.
An example of change that feels more unstable / inconsistent (to me) might be, say, the lore related Echo - what it is, what it comprises, why it provides immunity from tempering, and the implications on the world-setting. That feels less than smooth (to me) 1.x to 6.0 (especially between 4.0 to 6.0).