Move from a local world to shared multiplayer play.
Back up your Bedrock server, copy a verified existing world into its worlds folder, set level-name correctly and test the server before inviting players.
Back up the server, upload a verified Java world, point level-name to the correct folder, then test the world before players rejoin.
Migrate a Java server with its world, player data and settings safely. Back up, match versions, test privately and only then switch players over.
Verify the local world, keep two identifiable copies, match Java or Bedrock, and give a Realm owner a safe upload handoff before any slot is changed.