Over at Booklist's Likely Stories, Keir Graff has some worthy thoughts about reviewers' changes of mind or heart. We grapple with that in a mild way when we construct our Fanfare list every December--some book someone was crazy about doesn't seem so great anymore, and another star bites the dust; a book that was not starred in the first place is giving off more of a glow.
While the Horn Book (and I'm guessing the other journals) will only "take back" a review if it is shown to be factually inaccurate, any reviewer will frequently, over time, change his or her mind about a book. This can partly be ascribed to mood, but mostly I think it is because the reviewer has gone on to read more books and thus acquire a larger and different context to place the reviewed book in. Every book you read changes the way you read the one before it as well as the one you read after.