On an unsharded database you can update a document without having to pass in the _id as a value in the update document. The new document will retain the old _id. For sharded collections if you want to update a document you have to include the _id (or whatever your shard key is) or you will get an error.

use admin
db.runCommand(

{"enablesharding" : "test"}

);
db.runCommand( { shardcollection : "test.coll", key : {_id : 1} } );

use test
db.coll.insert(

{ "_id": "theid", "num" : 1}

)
db.coll.update(

{ "_id": "theid"}

,

{"num" : 2}

)

results in:

valid shard key must be in update object for collection: test.coll