Can You Downgrade a Minecraft Server?
Why downgrading a Minecraft world is dangerous and what to do instead.
The Short Answer
No, you cannot safely downgrade a Minecraft world. Minecraft uses a system called DataFixerUpper (DFU) to upgrade world data when a new version loads it. This process is one-way - there is no "DataFixerDowner."
If you load a world that has been touched by Minecraft 1.21 on a Minecraft 1.20 server, the world will either crash the server, corrupt blocks, or silently turn new blocks/items into air.
This applies to all world files: the Overworld, Nether, and End dimensions. It also applies to player data - a player who joined on 1.21 may have data that 1.20 can't read.
When Downgrade Is Relatively Safe
There's one scenario where downgrading is safe: you updated to a new version, realized there's a problem, and no players have joined yet. If no new chunks were loaded and no new items were generated, the world data hasn't been upgraded.
In this case, restore the backup you took before the update and select the older jar in Settings.
But the moment any player joins and loads chunks, DFU runs and the world is permanently upgraded. There's no way to undo this.
What to Do Instead
Restore from a backup: if you have one from before the update, restore it from the Backups tab. This is the only safe way to go back.
Put the server back on the old build: on GameGrid the way to run a specific version is the "Server JAR File" dropdown in the Settings tab. Upload the jar you want through the Files tab or SFTP and select it there; the setting is restart-required, so saving it restarts the server. There is no version dropdown anywhere else, and a plain restart does not change the build.
Generate a new world: with no backup, start fresh on the old build. You lose the builds and player data but the server is stable.
Use ViaVersion: this lets players on newer clients join an older server. It is a Bukkit plugin, so it runs on Paper ONLY - Vanilla, Fabric, Forge and NeoForge cannot load it. It does not downgrade world data; it only translates the connection.
Why This Happens
Each Minecraft version can add new block types, item types, entity types, and data formats. When a newer version loads an older world, DFU converts the old data to the new format.
The new format includes data structures and IDs that the old version doesn't know about. When the old version tries to read this data, it either crashes or misinterprets it.
This is why backing up before any version update is critical. There is no undo button for world data upgrades.