Minecraft wiki entry for Creating_behavior_packs, associated text: DO NOT CHANGE THIS NUMBER! As the Minecraft JSON updates, you will need to update your JSON files to match what was added. However, if you do update your JSON, you will need to change this number to whatever the new number is. Alternatively, you can copy the new version of the JSON from the updated Vanilla files and then copy over the specific behaviors you had changed in the old version.