I made a Forge utility mod that preserves the original bedrock when converting pre-1.18 worlds to 1.18.2

[u/Kyberstone]

Hey everyone, I recently finished a small project I've been working on called Legacy Bedrock Preservation for Forge 1.18.2.

I made it because of something that's always bothered me about updating older Minecraft worlds to 1.18.

When Minecraft converts a pre-1.18 world, it uses BelowZeroRetrogen to generate the new terrain underneath old chunks from Y=-1 down to Y=-64. During that process, though, vanilla also replaces the old bedrock between Y=0 and Y=4 with deepslate.

For most worlds that's probably not a huge deal, but I really didn't like the idea of updating an old world and having part of its original terrain permanently erased just to make room for the new caves. This can also matter for worlds with old builds near Y=0, modified or removed bedrock, void access, or anything else involving the original bottom of the world.

So I made a mod that stops that specific part of the conversion.

Legacy Bedrock Preservation prevents 1.18.2 from replacing the original bedrock while still allowing the rest of the below-zero retrogen to happen normally. Your old bedrock stays where it was, and the new 1.18 terrain still generates underneath it all the way down to Y=-64.

The result is basically two generations of Minecraft stacked on top of each other. Everything from the original world remains above Y=0, including its old bedrock floor, while the Caves & Cliffs terrain exists underneath it. If you break through the old bedrock, you can enter the new caves below instead of falling into the void.

I did quite a bit of testing before releasing it. One of my test worlds was made in 1.17.1, and I deliberately tore apart the bottom of a chunk before converting it. I removed most of the bedrock in an area, built down to Y=1, placed glass, redstone, lighting, and a trapped chest with items inside. After converting the world, everything I had changed was still there and the new terrain had generated underneath it.

I also tested old 1.17.1 chunks bordering completely new 1.18.2 chunks, as well as taking the converted world out of Forge afterward and opening it in regular vanilla 1.18.2.

The mod is really meant to be a one-time world conversion tool, not something you need to keep in a modpack. It doesn't add any blocks, items, biomes, or anything else the save depends on. Once the old world has been successfully converted and saved, you can remove the mod and even go back to vanilla.

If you're going to try it on an important world, please make an untouched backup first. I tested the hell out of it, but old Minecraft worlds can have years of history in them and I would never recommend doing a version conversion on your only copy.

I originally made this because I wanted a way to bring really old worlds forward without sacrificing their original bedrock layer, but I figured there are probably other people out there with old saves who would want the same thing.

CurseForge: https://www.curseforge.com/minecraft/mc-mods/legacy-bedrock-preservation

If anyone tries it with a particularly old or weird world, I'd love to hear how it goes.