Update a Minecraft server to a new version

1. The new version numbers#

On 2 December 2025 Mojang announced that "version numbers in the year 2026 will begin with ‘26’". The Minecraft Wiki sums up the format as year, drop number within the year, then a hotfix counter, so 26.3 is the third drop of 2026 and 26.3.1 would be its first hotfix. The last version with the old "1." prefix was 1.21.11.

What decides whether a player can join is the protocol number, not the name. The wiki: "Client and server versions with the same protocol version are compatible with each other."

So the three 26.1 releases can play together, while a 26.3 player cannot join a 26.2 server. The multiplayer list marks such a server "Incompatible version!", and the wiki lists the join messages as "Outdated client" when the server is newer and "Outdated server" when the player is.

2. Before you change anything#

• Back up. Paper's update guide starts with the backup and calls it "the most important step, and yet the most frequently skipped." Copy the whole server folder: worlds, configs, and plugins or mods.
• Check Java. From 26.1, Mojang's notes say "The game now requires Java 25"; 1.20.5 to 1.21.11 needed Java 21. Paper lists the same split. On a Java 21 runtime our Forge server for 26.3 never got going: Unrecognized VM option 'UseCompactObjectHeaders', then Error: Could not create the Java Virtual Machine.
• Check every plugin or mod. Look for a release that supports the new version before you switch. The jump to 26.1 was the hard one: Paper warned that plugins using Spigot-mapped internals "will not work on 26.1, no matter if you are using Paper or not."
• Wait for a stable build. Paper's first builds for a new drop are experimental, and its stable builds came 26 to 40 days after each 2026 drop. Paper is blunt: "We emphatically do not recommend using unstable builds or auto-updaters within production environments."

3. Updating Paper#

Paper's own steps: download the new jar, stop the server, replace the old jar with the new one under the name your start command uses, start it, and watch the startup log. Paper says not to replace any jar in a running server.

Moving a world from 1.21.x to 26.x also moves its folders. From 26.1, Mojang keeps each dimension under dimensions/minecraft/, and Paper dropped its separate world_nether and world_the_end folders. When our Paper 26.3 server first loaded a world made on Paper 1.21.11, it stopped to warn before touching anything:

[WorldFolderMigration] World storage migration is required during startup.
[WorldFolderMigration] If you do not have a backup: interrupt the server now. Use Ctrl+C, your panel kill function, etc.
[WorldFolderMigration] Continuing in 30 seconds...

Thirty seconds later it moved the Nether and End into the main world folder, upgraded all three, and removed the old folders. If you do not have a backup when you read that warning, interrupt the start within those 30 seconds, as the warning says.

4. Updating Vanilla#

Mojang publishes one server jar per version; the download page offers only the current release. Replace server.jar with the new one and start the server. Our Vanilla server, moved from 1.21.11 to 26.3, printed Starting upgrade for world "world" and Upgrade done for world "world" before it finished starting, and the dimensions moved under world/dimensions/minecraft. The --forceUpgrade option upgrades the whole world at once on startup; Paper says it "should rarely ever be used", because Paper upgrades chunks as they load.

5. Updating Forge#

Every Minecraft version has its own Forge line (1.21.1 is Forge 52, 26.3 is Forge 66), so a Forge update is a fresh install of the new Forge, and every mod needs a release for the new Minecraft version. Old mods cannot stay: our Forge server, moved from 1.21.1 to 26.3 with its 1.21.1 mods still in the mods folder, crashed with a mixin error instead of a plain version message:

Mixin apply failed balm.forge.mixins.json:EntityMixin -> net.minecraft.world.entity.Entity

Starting the world without its mods is no way round it. On our 1.21.1 server, when a world that used Waystones started without it, Forge warned:

Forge Mod Loader detected missing registry entries.
There are 99 missing entries in this save.
If you continue the missing entries will get removed.

On 26.3 the same kind of start printed only Missing data pack mod:waystones, so do not wait for a warning. Install the 26.x release of every mod that adds anything to the world (blocks, items, mobs, biomes or enchantments) together with the new Forge, before the world's first 26.x start. Our test world had no modded blocks in it, so we could start it on 26.3 with the old jars moved into a subfolder, which Forge does not read, and it upgraded; with a world you care about, try that only on a copy. The Forge mods guide shows the errors to expect as the new jars go in.

6. There is no going back#

Mojang's notes for the 26.1 snapshot that changed the world format put it plainly: "After upgrading, you will not be able to load your world in an older version at all." Paper adds: "Downgrading your world is not supported, so make sure to use the latest supported version of Paper." When we started Paper 26.2 on a world saved by 26.3, it logged:

Server attempted to load chunk saved with newer version of minecraft! 5023 > 4903

and stayed at Preparing spawn area: 6% until we killed it. The region files were not written, so setting the server back to the version that saved the world is the first thing to try, and the backup taken before the update is the fallback. Paper documents a flag that skips the check and calls it "highly not recommended, completely unsupported and may permanently corrupt your world". For players, Mojang's way to reach an older server is a launcher installation for that version, with a warning not to open their own worlds in it.

Where ForgeWorld Gaming fits#

Every ForgeWorld Gaming Minecraft server picks its version on the Startup tab: Minecraft Version and Build Number on Paper, Server Version on Vanilla, and Minecraft Version, Build Type and Forge Version on Forge. They take effect only when the server is installed or reinstalled. On our Vanilla test server, changing Server Version and restarting still started 26.3; after Reinstall Server on the Settings tab it ran the version we chose, with the world and server files kept. That also means a server set to latest stays on the version it was installed with until you reinstall. Paper's reinstall kept the old jar as server.jar.old; Vanilla's replaced it. Take a backup on the Backups tab before you reinstall, as the panel advises.

Two settings can install something other than what you meant. On Forge, set Build Type to latest whenever you change Minecraft Version: with recommended, a version that has no recommended Forge build and shares its prefix with others, such as 26.1 today, leaves the install script without a Forge build to download, and the server stays where it was. On Paper, a Minecraft Version that Paper never built, such as 26.1, makes the install fall back to Paper's newest version, and a Build Number of latest takes Paper's newest build for that version, experimental or not. Either way, read the version line the console prints at the next start before players join.

New Paper and Vanilla servers start on the Java 25 image, which also ran Paper 1.21.11 in our test. Forge servers start on Java 21 for their default 1.21.1, so before moving a Forge server to 26.x, choose Java 25 in the Docker image selector on the Startup tab; on our server that cleared the Java error above, and the server ran 26.3 once the old mods were out of the way too. To interrupt a start, whether during Paper's 30-second migration warning or a hang like the downgrade above, press Stop and then Kill, which the Console tab offers once a stop is in progress. Last checked 2026-10-20.

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