DOCS-599 change shard key page faq entry

Author: Sam Kleinman

source/administration/sharding.txt

@@ -146,6 +146,8 @@ use the following procedure as a quick starting point:
       :term:`primary shard` for that database, where MongoDB stores all data
       before sharding begins.
 
+.. _sharding-administration-shard-collection:
+
 #. Enable sharding on a per-collection basis.
 
    Finally, you must explicitly specify collections to shard. The

source/faq/sharding.txt

@@ -36,6 +36,34 @@ How does sharding work with replication?
 To use replication with sharding, deploy each :term:`shard` as a
 :term:`replica set`.
 
+Can I change the shard key after sharding a collection?
+-------------------------------------------------------
+
+No.
+
+There is no automatic support in MongoDB for changing a shard key
+after :ref:`sharding a collection
+<sharding-administration-shard-collection>`. This reality underscores
+the important of choosing a good :ref:`shard key <shard-key>`. If you
+*must* change a shard key after sharding a collection, the best option
+is to:
+
+- dump all data from MongoDB into an external format.
+
+- drop the original sharded collection.
+
+- configure sharding using a more ideal shard key.
+
+- :ref:`pre-split <sharding-administration-pre-splitting>` the shard
+  key range to ensure initial even distribution.
+
+- restore the dumped data into MongoDB.
+
+See :dbcommand:`shardCollection`, :method:`sh.shardCollection()`,
+:doc:`/administration/sharding`, :ref:`sharding-internals-shard-key`
+:doc:`/tutorial/deploy-shard-cluster`, and :issue:`SERVER-4000` for
+more information.
+
 What happens to unsharded collections in sharded databases?
 -----------------------------------------------------------