It should work with an existing world, but as always, make backups! As for integration. This time round instead of the "harsh" borders we've gotten before (wh

It should work with an existing world, but as always, make backups! As for integration. This time round instead of the "harsh" borders we've gotten before (where new and old chunks meet, often resulting in sheer cliffs due to height differences) the borders will be blended and smoothed. Your old chunks will also be upgraded, the bedrock that was at Y0-5 will turn into the deepslate block, and you get the full 0 to -64 Y generated underneath. Ethoslab, a youtuber with a very old world save, is on 1.18 now, and they've got one of the new lush caves in the new negative Y generation under their base.