Direct answer

level-seed is used when a world is generated. Changing it does not rewrite chunks that already exist. To start fresh with a new seed, stop the server, back up the old world, set the seed, and use a new level-name or move the old world out of the way before starting again.

Set the seed

level-seed=123456789

You can use a numeric seed or another value accepted by the Minecraft version you are running. Leave it blank for a random seed.

Create the new world safely

  1. Run stop.
  2. Back up the current world.
  3. Edit level-seed.
  4. Change level-name to a new folder name, for example world-new, or move the old world folder away.
  5. Start the server and verify the new world.

Why existing terrain does not change

Generated chunks already contain saved block and biome data. A new seed only affects terrain generated under the new world's data. Mixing seeds in one existing world can create visible borders or unexpected transitions when new chunks are generated.

Keep the old world until you are sure

Do not permanently delete the old world just to force regeneration. Rename or archive it first so you can recover player builds, inventories, or structures if the new world is not what you expected.

Official references

These references were checked while preparing this guide. Version-specific behavior can change, so use the project documentation for your exact release when a detail differs.

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