Currently, replacement style updates target using the update rather than the query. We should change this to target using the query instead.
As of today, we do not allow users to change the shard key for a document. If shard 1 owns the chunk with range (shardKey : min, shardKey : 500) and shard 2 owns the chunk with range (shardKey : 500, shardKey : max) and a user sendsthe update will be sent to shard 2 and we return We should actually get an ImmutableField error.
Once we allow a user to update the shard key for a document (PM-1163), the above update should cause the document to change shards. However, since the update would be sent to shard 2, shard 1 will never actually see this update and the update will not be applied.