I still think it's a pity that the Harry Potter books were Americanized for US publication; when I was a kid I liked reading British kids' books, and doing so taught me some things about the differences in culture and language between the US and UK. (I even learned about the wacky pence/shillings/pounds system before they regularized it.)
But it turns out that the books haven't had all that many changes made between editions, as you can find out by perusing the differences between the UK and US versions (halfway down the page, under "Language Resources").
That list indicates that, as a couple of people had already suggested to me, I was wrong in thinking that the uses of "uptight" and "bummer" in book 4 might have been American insertions. (I was also wrong in thinking that nobody uses those words any more.) I was right about "English muffins" being a replacement for "crumpets" in book 1, though. (Which is particularly silly given that five pages later they use the word "crumpets.")
Thanks to Amy S. for the pointer. As Amy put it: "Aren't obsessive fans great?"
(Oh, dear, I just noticed that I know three different people to whom Amy S. could refer. And two of them have the same middle initial.)