Yeah, I've heard this before. The idea that some pronunciations changed in British English but were preserved in the USA. And the idea that "gotten" is an Americanism when it used to be standard English.A friend of mine who teaches linguistics told me that Chaucer probably sounded a bit like a modern (to be vague) American. And that the American accent (sorry to be vague) is closer to how middle english sounded than the modern English accent.
I think it's crazy how far the Aussie and Kiwi accents have diverged from British English in such a short time. I guess you can hear a lot of Cockney in Australia accents but the accent in NZ sounds almost like a cross between Aussie and South African.