[thaumasiotes]

> I wonder how many generations of retellings it takes for us to notice significant differences.

That's not really a sensible question. Compare the 17th-century European story of Cinderella to the 9th-century Chinese story of Ye Xian: https://en.wikipedia.org/wiki/Ye_Xian

The story stayed nearly identical for a period of many centuries. Significant differences could have been introduced at any point, but they weren't.