habitica/migrations/command-line/facebook_to_local.js

let oldId = '';
let newId = '';
let newUser = db.users.findOne({_id: newId});

db.users.update({_id: oldId}, {$set: {auth: newUser.auth}});

// remove the auth on the new user (which is a template account). The account will be preened automatically later,
// this allows us to keep the account around a few days in case there was a mistake
db.users.update({_id: newId}, {$unset: {auth: 1}});