If you open this custom world 21w42a-height4064.json in 21w43a after you generated it in 21w42a results in all terrain below -64 disappearing and any blocks being placed above 320 / max_y.
- Open Minecraft in 21w42a
- Create a world using the custom world settings in 21w42a-height4064.json
Enter spectator mode, and observe that the world bottom is lower than Y=-64 and world top is higher than Y=320 (e.g. by placing a block above it)
- Open Minecraft in 21w43a
- Play the world you created previously to upgrade it
The terrain previously generated below Y=-64 has been removed, and the bottom of the world is now found at Y=-64.
Blocks above Y=320 are removed, and the top of the world is now found at Y=320
- The upgraded world would not have all terrain below Y=-64 and above Y=320 removed, the dimension's bounding boxes / min_y and height should stay unchanged.