Minecraft wiki entry for Caves_%26_Cliffs, associated text: The biomes and terrain from old and new chunks now seamlessly blend, preventing the hard cut borders from before. Any player-made structures in these chunks may either be intact, buried, or deteriorated.
The old bedrock layer between Y=0 and Y=4 in old chunks gets replaced with deepslate.
A new bedrock layer is placed between Y=-64 and Y=-60.