The only way to switch an existing world to Hardcore mode is to use a third-party app to edit the game's files. How to change game modes in 'Minecraft: Java Edition' 1.

The Minecraft Bedrock Edition supports crossplay on Android, iOS, Xbox, PlayStation, Nintendo Switch, and more. Bedrock players also have access to Realms, a multiplayer server run by Minecraft.

Minecraft Java Edition players can access older versions through the games file history with a few easy steps. Bedrock players, unfortunately, cannot.