updates and clarifications to upgrade process

Author: Sam Kleinman

source/release-notes/2.4-upgrade.txt

@@ -26,8 +26,16 @@ This procedure applies only to upgrades from version 2.2. Earlier
 versions of MongoDB do not correctly handle epochs.
 
 While the upgrade is in progress, you cannot make changes to the
-collection metadata. For example, you cannot add shards, drop databases,
-or drop collections, nor make other metadata changes.
+collection metadata. For example, you cannot perform operations that
+add shards, drop databases, or drop collections, or modify the
+metadata in any other way.
+
+.. warning:: 
+   
+   Once you upgrade to 2.4 and complete the upgrade procedure **do
+   not** use 2.0 :program:`mongod` and :program:`mongos` processes in
+   your cluster. 2.0 process may re-introduce old metadata formats
+   into cluster metadata.
 
 Sharded Cluster Upgrade Procedure
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -88,13 +96,10 @@ procedure.
    option.
 
 Once you have upgraded, *do not* introduce version 2.0 MongoDB
-processes into the sharded cluster. This can introduce in
-metadata problems. 
-
-.. is the following true? 
-
-   This metadata change and the upgrade mechanism will help prevent
-   errors caused by cross-version incompatibilities.
+processes into the sharded cluster. This can reintroduce old metadata
+formats into the config servers. The metadata change made by this
+upgrade process will help prevent errors caused by cross-version
+incompatibilities in future versions of MongoDB.
 
 .. _upgrade-cluster-resync:
 
@@ -164,7 +169,7 @@ To resync the config servers:
 
       mongo <2.2 mongos port>
 
-   Then, use the following operation to reset the balancer process:
+   Then, use the following operation to reset the upgrade process:
 
    .. code-block:: javascript