db.users.update(
    {items:{$exists:0}},
    {$set:{items:{weapon: 0, armor: 0, head: 0, shield: 0 }}},
    {multi:true}
);

db.users.find().forEach(function(user){

    var updates = {
        // I'm not racist, these were just the defaults before ;)
        'preferences.skin': 'white',
        'preferences.hair': 'blond',

        'items.head': user.items.armor,
        'items.shield': user.items.armor,
    }

    db.users.update({_id:user._id}, {$set:updates});

})