You upgraded, something did not work, and now you want to go back. The server software downgrades easily. The world does not — and that is the whole difficulty, because the world is the part you care about.
What you are seeing
- The server boots, starts loading the world, and crashes. Every time.
- Errors mentioning chunk or region data during startup.
- High CPU with the server never reaching its «Done» line.
- It started immediately after a version change.
Why it happens
Opening a world in a newer Minecraft version converts it, and that conversion only goes forward. New blocks, new data formats and a new region layout have no meaning to an older server core, so it fails on the chunks it cannot read and restarts into the same failure.
The only route back that keeps your world
Restore a backup taken before the upgrade. That copy is still in the old format, so the old server version opens it normally, and you lose whatever happened between the backup and now — nothing else.
This is why the backup step in every version-change article is not decoration. It is the entire mechanism by which a version change is reversible.
Restoring it
- Stop the server
Do not restore over a running server. It holds the world in memory and will write it back over what you just restored.
- Set the version back to the old one first
In the Startup tab, and check the jar filename matches the file that will exist. Restoring a world and then starting a newer core converts it again immediately.
- Restore the pre-upgrade backup
The most recent one from before the change. If you are not sure which, check the dates against when you changed the version.
- Start and watch the console
It should reach its «Done» line. If it still crashes on chunks, the backup is from after the upgrade — try the one before it.
No tool converts a world downwards reliably. At that point the honest options are two: stay on the newer version, which keeps everything, or delete the world and generate a fresh one on the old version, which keeps nothing. Wiping the world is not a downgrade — it is starting over, and it is worth saying that plainly before anyone deletes anything.
Five minutes, and it converts every future upgrade from a one-way door into a decision you can reverse. It is the same habit the plan-change and reinstall articles ask for, for exactly this reason.
The server starts on the old version, reaches «Done», and you can walk through builds you recognise with no chunk errors in the console.