Database
[{"_id":1,"Region":"Northern","Items":[{"ItemId":NumberInt(25),"Name":"Widget","ProductType":"15","ItemIdLegacy":"ca-000037"},{"ItemId":NumberInt(30),"Name":"Gizmo","ProductType":"15","ItemIdLegacy":"ca-000038"},{"ItemId":NumberInt(35),"Name":"Thingy","ProductType":"15","ItemIdLegacy":"ca-000039"}]},{"_id":2,"Region":"Northern","Items":[{"ItemId":NumberInt(25),"Name":"Widget","ProductType":"15","ItemIdLegacy":"ca-000037"},{"ItemId":NumberInt(30),"Name":"Gizmo","ProductType":"15","ItemIdLegacy":"ca-000038"},{"ItemId":NumberInt(35),"Name":"Thingy","ProductType":"15","ItemIdLegacy":"ca-000039"}]}]
Query
db.collection.update({"Items.ItemIdLegacy":"ca-000038"},{$set:{"Items.$.ProductType":20}},{multi:true})