Moving to a newer Minecraft version is routine. Moving to an older one is not, and the difference matters because one of them can destroy a world permanently.
Why it happens
Worlds are upgraded in place. When a newer version opens an older world it rewrites the region files into the new format, and that conversion only runs forwards. An older version opening a newer world does not know how to read it.
What to do
- Take a backup first
Before any version change, make a backup and download it. This is the single step that turns a bad upgrade into an inconvenience instead of a loss.
- Stop the server
Change the version while it is off. Swapping the jar under a running server produces corrupted region files.
- Change the version in the panel and start
The new jar is fetched on the next boot. The first start after a version change is slower because the world is being converted.
- Check your plugins and mods
They are built against a specific version. After a jump, expect some to fail to load until you update them — the console names each one.
The console reaches Done and you can join. Visit a few pre-existing builds to confirm the world converted cleanly.
Going down a version is the dangerous direction. A world already converted upwards will usually not open, and when it does open it can lose chunks. Restore from a backup taken before the upgrade instead.