How to Switch a Project Zomboid Server Between Build 41 and 42

To switch a Project Zomboid server between Build 41 and Build 42, back up Zomboid/Saves/Multiplayer/<servername> and the Zomboid/Server/<servername>* files, reinstall app 380870 on the other branch (-beta legacy41 for Build 41, no beta for Build 42), and start it with a different -servername. Worlds do not move between builds, so the new name gives you a fresh world and leaves the old one untouched for later. Players switch their Steam branch to match.

Project Zomboid survivors in Knox County, a world that stays on the build it was made on

What you can and cannot move

Thing	Between Build 41 and Build 42?
The world (map, bases, loot, vehicles)	No. The Indie Stone says Build 41 saves are not compatible with Build 42
Characters and skills	No, they are part of the world
Server settings (`<servername>.ini`)	Partly. Copy the values you care about (password, max players, PvP) by hand
Sandbox settings (`<servername>_SandboxVars.lua`)	No. Build 42 has new and changed sandbox options, so start from its defaults
Accounts and whitelist (`db/<servername>.db`)	Recreate them on the new world
Mods	Only mods whose Workshop page lists the build you are moving to
Keeping both worlds	Yes, under two server names, or on two servers

What goes where

All of these live in the Zomboid folder of the user running the server (C:\Users\<you>\Zomboid\ on Windows, ~/Zomboid/ on Linux), or in the folder set with -cachedir.

File or folder	What it holds	Build specific?
`Saves/Multiplayer/<servername>/`	The world	Yes, it only loads on the build that made it
`Server/<servername>.ini`	Ports, password, mods, max players	Shared format, but the `Mods=` line differs
`Server/<servername>_SandboxVars.lua`	Zombie, loot and world rules	Yes, options differ
`Server/<servername>_spawnregions.lua`	Spawn points	Map dependent
`db/<servername>.db`	Accounts, admins, whitelist	Tied to the world you use it with

Both builds use the same Zomboid folder. That is what makes the next section matter.

The gotcha: same server name, wrong build

The server picks its world and settings by server name, not by build. If you reinstall on Build 42 and start with the same -servername (often servertest), the new server goes straight to your Build 41 world folder and sandbox file. At best it fails to load with errors. At worst it starts writing Build 42 data into a world Build 41 can no longer read cleanly, and your old save is damaged too.

The fix is simple: one server name per build. For example keep servertest for the Build 41 world and start Build 42 as b42world. Build 42 then creates Saves/Multiplayer/b42world/ and fresh b42world.ini and b42world_SandboxVars.lua with its own defaults, and the Build 41 files are never touched.

Before you start

Back up first. Copy Saves/Multiplayer/<servername>/, the three Server/<servername> files and db/<servername>.db somewhere safe.
Note your settings. Write down the password, MaxPlayers, PvP and any sandbox values you want again.
Check every mod. Open each mod's Workshop page and check it lists the build you are switching to.
Tell players. Everyone needs the matching Steam branch, or they will not see the server.

Step 1: Stop the server and back up

Type quit in the console so the world saves, then copy the files above off the server.

Step 2: Reinstall on the other branch

For Build 41:

steamcmd +force_install_dir /path/to/pzserver +login anonymous +app_update 380870 -beta legacy41 validate +quit

For Build 42, run the same command without -beta legacy41. The validate matters, because it replaces files from the other build instead of mixing them.

Step 3: Start with a new server name

Start the server with a name you have not used, for example:

./start-server.sh -servername b42world

The first start creates the new world and settings files. Stop it again once it has finished starting.

Step 4: Set up settings and mods for this build

Open Server/b42world.ini and put back your password, MaxPlayers and other values. Then add mods for this build only:

	Build 41	Build 42
`WorkshopItems=`	`2169435993;2392709985`	Same format
`Mods=`	`ModA;ModB`	`\ModA;\ModB`, a backslash before each Mod ID
Mod versions	B41 releases	Many mods ship a separate B42 Workshop item or Mod ID

Our Project Zomboid mods guide covers both formats. Start the server and check the console loads every mod.

Step 5: Players switch their Steam branch

Right click Project Zomboid in Steam and open Properties.
Go to Game Versions & Betas.
Pick legacy41 for Build 41, or the default (none) for Build 42.
Let Steam download, then join.

Common problems

Server starts with errors loading the world: it is opening a world from the other build. Use a new server name, or restore the backup and switch back.
Players cannot see the server: they are on the other Steam branch. The server list only shows servers on your own build.
"Server version is different" when joining: the branches match but the versions do not. One side needs to update, see updating a Project Zomboid server and its mods.
Mods download but do nothing on Build 42: the Mods= entries are missing their backslash, or the mod has no B42 version.
Sandbox options look wrong: you copied the Build 41 SandboxVars.lua across. Delete it and let Build 42 write a fresh one.
Old world gone after switching back: the same server name was used on both builds. Restore it from the backup.

Switching on GamezServers

On a GamezServers Project Zomboid server, open Settings, pick Build 42 or Build 41 (legacy) under Server type and press Switch. We back up the server first, then reinstall it on the right branch, and your files stay on disk. Before the first start, change Save name in Settings too: a new name starts a separate world and its own settings files in .cache/Server, and the old world stays on disk for when you switch back. The Mod IDs setting gets the Build 42 backslash added for you, and left off on Build 41. Not sure which build to pick? See Build 41 vs Build 42 for multiplayer.

FAQ

Can I move my Build 41 world to Build 42?

No. Build 41 saves are not compatible with Build 42. Keep the old world on a legacy41 server and start a new one on Build 42.

Can I keep both a Build 41 and a Build 42 world?

Yes. Give each its own server name, and only the one matching the installed build will load. Switch the branch and the name together.

Do I need to reinstall to change builds?

Yes. The server files come from a different Steam branch, so update app 380870 with or without -beta legacy41 and validate.

Do Build 41 mods work on Build 42?

Only if the author released a Build 42 version. Many mods publish a separate item or Mod ID for B42, so check each Workshop page.