var oldId = "",
    newId = "", 
    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}});