Height issue when converting from high version to low version

[Fomalhaut-i]

Chunker Version

Chunker-1.15.0-windows-x86

Input World Version

java 1.20

Output World Version

java 1.12

Platform / Operating System

Win 11

Issue Description

I encountered a critical issue when converting a higher-version Minecraft world to an older Java Edition version using Chunker.
Newer Minecraft versions support negative Y coordinates, allowing blocks to exist below Y=0. However, many older Java versions only support Y ≥ 0, and the area below Y=0 is considered void where blocks cannot exist.
During conversion, all blocks below Y=0 are completely removed(because in the void), while blocks at or above Y=0 are preserved correctly.This results in massive parts of the world—especially underground structures and terrain—being lost after conversion.
I think when converting from high to low versions, a y-axis offset should be added to make the height coordinates of the entire world shift upwards

[FormallyMyles]

There are no current plans to do this, as mapping 0 -> 0 makes the most sense for keeping structures which use height data working / flat maps on a usable level. For this reason it isn't recommended to downgrade worlds below 1.16.5 since they lose data.

You could optionally use another tool to move your entire world up / down before converting with Chunker, though I'm unsure of a tool that can move the entire world height.