var query = {
  '$or': [
    {'items.gear.owned.weapon_special_0': true},
    {'items.gear.owned.armor_special_0': true},
    {'items.gear.owned.head_special_0': true},
    {'items.gear.owned.shield_special_0': true}
  ]
};

db.users.find(query, {'items.gear.owned':1,backer:1}).forEach(function(user){
  var owned = user.items.gear.owned;
  var tier = (user.backer && user.backer.tier) || 0;
  if (tier < 70) delete owned.weapon_special_0;
  if (tier < 45) delete owned.armor_special_0;
  if (tier < 45) delete owned.head_special_0;
  if (tier < 45) delete owned.shield_special_0;


  db.users.update({_id:user._id}, {$set:{'items.gear.owned':owned}});
});