Change your Minecraft version or mod loader

Your Minecraft server runs one piece of server software, sometimes called the jar or mod loader, at one Minecraft version. The JAR Manager tab changes either one and fetches the new jar for you, with nothing to upload yourself.

Which server software should I pick?

Before you start

1. Plan on a backup. The JAR Manager takes one for you before it reinstalls (Take a backup first, ticked by default), or open the Backups tab and click Create backup. If your plan's backup slots are full, a new backup replaces the oldest one. How to create and restore backups covers rolling back.
2. Warn your players. The server goes offline for the reinstall and stays off until you start it again.
3. Check for a modpack. If one is installed, the JAR Manager names it at the top. Applying any change here, even just a new version, removes the pack, though its mods stay in the server folder. To update or swap the pack itself, use the Modpacks tab instead.

Change the version or software

1. Open your server from the dashboard. In the left rail, expand More and choose JAR Manager (on a phone, pick JAR Manager from the section menu at the top of the panel).

2. Under Server software, pick Vanilla, Paper, Purpur, Fabric, Forge or NeoForge. Your current choice is tagged Installed.

3. Under Minecraft version, pick the version you want.

4. Click Apply change. The Reinstall server? box checks your plugins or mods against the new choice and lists any with no build for it. Leave Take a backup first ticked, and Disable the ... that won't run (kept, not deleted) if it's offered, so those files are renamed rather than loaded.

   

5. Click Confirm & reinstall. The panel takes the backup, downloads the new jar and switches Java to match. Your world and player data stay, but config files may be reset. Open the Console to follow the install. The reinstall clears the old console log first.

6. When the Reinstalling badge at the top of the panel disappears, click Start. The server is ready when the Console shows Done (…s)!.

   

How long it takes, and what stays

We switched one Diamond plan server (4 GB RAM) between Vanilla, Fabric and Forge on Minecraft 1.21.1, then down to Vanilla 1.20.1, on 14 October 2026:

What survived every switch:

• The world. Every start after a switch loaded the existing world folder. Only the very first start made a new one.
• server.properties. The test MOTD we set before the first switch was still there after the last, so the Config Editor settings carried over. Loader config files in config/ can be reset, as the panel warns.
• The mods folder. Nothing in it is deleted. Mods with no build for the new choice are only renamed to .disabled if you tick that box, and a switch to Vanilla leaves them on disk, unused.
• The jar it replaced, kept as server.jar.bak. The JAR Manager then shows Previous jar kept with a Restore previous jar button.

Update Paper or Purpur on the same version

Paper and Purpur keep releasing builds for a Minecraft version after it ships. When a newer build exists, the JAR Manager shows an Update available row.

1. Take a backup in the Backups tab. The update keeps your world, but config files may be reset.
2. Click Update build, then Confirm & update.
3. When the reinstall finishes, click Start and wait for Done (…s)! in the Console.

Which Java version does my server need?

Each Minecraft version needs a particular Java version. The JAR Manager sets it for you when it reinstalls:

Plugins and mods after a change

Plugins and mods are built for particular Minecraft versions, so some stop loading after a version change until you update them. Check the Plugins tab afterwards (it's called Mods on Fabric, Forge and NeoForge).

Plugins only run on Paper and Purpur, and most mods only run on the loader they were built for. After switching, add the equivalents with Install Minecraft plugins on Paper or Purpur or Install Fabric and Forge mods on your server.

Don't count on a crash to tell you a mod is on the wrong loader. In our tests Forge 1.21.1 started with three Fabric mods in mods and said nothing about them, and Fabric only printed Found 1 non-fabric mod: for a Forge mod. Neither loaded the other's mods, so let the switch disable them, or remove them in the File Manager, and install the new loader's builds.

If something breaks

To undo the change:

1. In the JAR Manager, pick the software and version you had, then click Apply change and Confirm & reinstall. Don't start the server yet.
2. When the reinstall finishes, open Backups and click Restore on the backup from Before you start. The panel starts the server once the files are back.

Keep that order. When we restored a Forge 1.21.1 backup while the server was still set to Vanilla 1.20.1, it wouldn't start: the files were Forge's, but Java was still the version 1.20.1 runs on, and the console showed java.lang.IllegalStateException: Current Java is 17 but we require at least 21.

If you only changed the version or build of Vanilla, Paper or Purpur, there's a quicker undo: stop the server and click Restore previous jar under Previous jar kept. It swaps server.jar.bak back in with the old version's settings, without a reinstall. After a switch between loaders, use the two steps above instead.

To keep the change and fix the problem, match what you see:

• The console shows No key dimensions in MapLike - the world's level.dat doesn't suit the new version or loader. See No key dimensions in MapLike: fix level.dat.
• A banner says the last reinstall didn't complete - click Reinstall on that banner. If it fails again, use its Contact support button.
• The server stops with Incompatible mods found! - a mod for the new loader is built for another Minecraft version or needs a mod you don't have. The lines under it name the mod. Update or remove it and start again (the messages, word for word).
• The console shows Current Java is 17 but we require at least 21 - the files and the Java version don't match, usually after restoring a backup made on other software. Reinstall the software the backup was made on in the JAR Manager, then restore it again.
• The console shows UnsupportedClassVersionError - a jar needs a newer Java than this Minecraft version runs on. If the error names a plugin or mod, update it to a build for your version or remove it in the File Manager. If it names the server jar itself, open a ticket from Support and paste the full error.