There are three species in the game:

• Human
• Zrilian - Breathes H2
• HEM Droid - Runs on batteries

To switch species in an existing save place a Spawn Point inside your base and set the spawn point. This is where the "new you" will spawn. You may want to drop your gear so your new character can use it.

After making these preparations, save the game and make a backup!!!

Now, open world.xml in a text editor and search for a ThingSaveData with a ClientSteamId that matches your steam64 id. Then remove the line with the ClientSteamId.

Save the file and load the save. You should spawn at the spawn point as a new species. Your old avatar will still be there.

    <ThingSaveData xsi:type="BrainSaveData">
      <ClientSteamId>1234567890</ClientSteamId> <--- remove this