This series was originally written in a "rolling present" mode. The first one is set in 1983; the third one takes place a few months later, but it's approximately 1990 and portable computers are hot; by the ninth, the characters are still teenage but they carry iPods.
Duane has started rewriting the series for a fixed time base: So You Want to Be a Wizard now begins in May 2008, and the rest follow from there. The "New Millennium Editions" are available through book 3, so far, as ebooks on the author's web site.
Is this what the series needs? It's not what I need, but I'm all grown up and stuff. I can certainly see the books being more accessible to young readers, now that Kit and Nita have cellphones like normal kids do. (And computer interfaces don't all look like DOS, and there are more than three Star Wars movies.)
Contrariwise, in twenty years it's going to look dated all over again. Or sooner -- maybe we'll all be wearing Q-Earrings in 2017. Who knows. All contemporary-setting fantasy has this problem, so it's just a question of how much time Duane wants to spend rewriting as opposed to writing new stuff.
There are a few (small) added scenes, quite a bit of updated slang, and some factual errors are fixed. (The gravity of Mars is correct now, thank you.) The only significant plot change is the bit about Olbers' Paradox; it has been gracefully withdrawn and replaced by a different spell. I rather regret it. No, the original scene made no sense whatsoever, cosmologically speaking... but it made a really lovely sort of no sense. It hit me right. The rewritten scene is way too on-the-nose. For me. I don't want to go telling the author to rewrite it again, that would be tacky, I'd rather have book 10 anyhow, right?
At any rate, this gave me (another) excuse to go back and read the early books in the series, so it's all good.