Harry Potter's creator loves to sprinkle fandom with data that isn't in the books—most significantly Dumbledore's sexuality, and most lately the house that Harry's son is sorted into. David Sims suggests she's gone Lucas on us.
As Lucas has shown, such an impulse can lead to larger changes that are detrimental to the original work. And given that much of the joy in falling in love with fictional characters comes from being able to envision new stories from them, by continuing to embellish her stories long after publication, Rowling is arguably chipping away at that imaginative freedom.
What he's getting at—and perhaps conflating—is that there are two things authors tend to do if they're not done when they're done.
1. Change an established detail.
2. Add a new detail.
The former is often called retconning ("retroactive continuity" as a verb) and is a common source of fan anguish.
The second doesn't seem to have a common descriptor (but often is found within prequels and sequels), is often beloved by fans, and isn't the same thing.
But it can be a bad thing, in that knowing these details changes the context of established events in a way that undermines or hurts the integrity of the story or its characters.
Sims's objections to the latter are quite profound, I think: "The more Rowling enhances and embellishes her Harry Potter universe, the less room she leaves for readers to fill in the gaps with their own imaginations."
This touches on the relationship between author and audience—and how easy it becomes too one-sided when an author never lets their audience close the book and dream.
But I don't think the Lucas comparison is apt, because the sheer deluge of both (1) and (2) in the Star Wars universe—and especially the hamfisted, inexplicable incompetence of their execution—makes his a special case.
The real problem with adding detail is when it comes in terrible packages: tweets, responses to emails, interview remarks and the like. These things are not stories. They're injections of data, plugging directly into our story receptors, but providing none of the emotional or spiritual satisfaction found in the deep hacky magic of storytelling.
When beautifully presented, things are different. Sometimes, a retcon fixes a ugly problem. Sometimes, a fresh detail creates a beautiful new one.
