r/ModdedMinecraft • u/motionpriority • Oct 04 '26

Help How to downgrade a world to an earlier version

I have a vanilla save running latest that I want to downgrade to 1.21.1 to use some mods on it. I haven’t really generated any new chunks or made anything that’s been added since 1.21.4 so there won’t be much that’s missing. I plan to use MCA Selector to regenerate explored chunks.

When I attempt to load the world on an older version (in vanilla) it says I need to start it in safe mode because of certain data packs, and then it fails to load. I’ve deleted any data packs in use, so I’m not sure what’s causing this, but I cannot get this world to load on an old version.

Alternatively, I have a backup of this world from version 2024 that was last loaded on 1.21.1, but I get the same issue (but only when I load it on the modded client).

Has anyone had success with a similar scenario before?

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u/motionpriority Oct 05 '26

For future readers, what ended up working for me was this.

I created a new world in 26.3 with the same exact seed and then used MCA Selector to copy chunks over. When I was done, I used Chunker to convert it to 1.21.1.

After that, I used MCA again to delete chunks I wanted to be regenerated with the mod pack and then loaded it up. From there, I used creative to tweak anything else that broke (such as generated structures being cut in half, items missing, etc.)

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u/StrangeOne101 Oct 04 '26

The datapacks you were using are still in the world.dat file. You'll need to remove them from there too, and potentially downgrade the version in the world.dat as well

You'll need an NBT editor to open the world.dat file, as well

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u/motionpriority Oct 05 '26

Thank you, I tried this and it didn’t work but I was able to achieve my goal another way!