threetwo:PRIMARY> db.version()
3.2.0

// Insert a document where x is a Double
threetwo:PRIMARY> db.foo.insert({x: 1})
WriteResult({ "nInserted" : 1 })

// Verify it's a Double
threetwo:PRIMARY> db.foo.find({x: {$type:1}})
{ "_id" : ObjectId("56829f8f891b45519f22028c"), "x" : 1 }

// Update to NumberInt
threetwo:PRIMARY> db.foo.update({ "_id" : ObjectId("56829f8f891b45519f22028c")},{$set: {x: NumberInt(1)}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 0 })

// Note nModified of 0, and field is still Double
threetwo:PRIMARY> db.foo.find({x: {$type:1}})
{ "_id" : ObjectId("56829f8f891b45519f22028c"), "x" : 1 }