what if they came and said that they wanted Animorphs to be rewritten? Would you do it? How early in the series would you start making major changes?
Personally, I'd likely start as early as book 27. Mainly because I would have wanted David to become a main villain, working for crayak. Crayak would give him back his morphing ability in the book after the Animorphs defeat the Howlers in book 26. Interetingly enough, that would wind up returning David during a Rachel narrated book, who is the Animorph David hates the most.
Of course, I'd also make small changes earlier, since some things were retconned or changed, but I'm talking more about major plot changes.