This ticket should incorporate

(1) updates to fields in a documents cluster key when the collection is not sharded with any of said fields (see if SERVER-60119 already enables this)

(2) updates to fields in a documents cluster key when the fields are included in the shard key  — for context, this may already work as it does with non-clustered collections.

How sharded collections handle shard key updates today (non-clustered):
The mongos sends an update, if the update modifies a document's shard key, an error gets propagated back to the mongos, and the mongos issues a delete for the preimage and insert for the postimage in a multi-document transaction. Clustered collections should behave the same as non-clustered collections in this way.