The fact that it changed, viewed from this distance seems undeniable. Clues to rhyming and assonance paint a very strong picture of both a written and a spoken shift, quite aside from the astounding rise of loan words, and inventive use of English.
The great vowel shift followed in due course. I'm less sure valley speak will be the enduring change but vocal fry seems to be sticking.
I'm unsure creolisation alters much. Would it change anything, beyond categorising?