The two timelines are vastly different, with the Nantucket series taking on the standard trope of transposition of modern day people into the distant past, and the Emberverse taking on the trope of the failure of all modern technology.
The Change transposes Nantucket Island into the past of several thousand years ago. Its ancient counterpart appears in its place of modern day 1998. This transposed island first appears in the Emberverse The Sunrise Lands novel, with ancient Paleo-Amerinds and ancient old-growth forests. The 1998 Nantucketers encounter the world of the Trojan War era, pre-Celtic Europe, pre-Phoenician Mediterranean, and the Shang Dynasty China.
The Change event occurs in 1998, as it is the time when the first novel in the universe was written, Island in the Sea of Time of the Nantucket series. All further novels reference this date due to that fact.