[{_id:"123456",id:"123",inventory:[{id:"foo",count:0},{id:"bar",count:3}]}]

db.collection.update({},[{$set:{inventory:{$map:{input:"$inventory",in:{$arrayToObject:{$map:{input:{$objectToArray:"$$this"},in:{k:{$cond:[{$eq:["$$this.k","count"]},"price","$$this.k"]},v:"$$this.v"}}}}}}}}])