Index: doc/src/sgml/pgupgrade.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/pgupgrade.sgml,v
retrieving revision 1.10
diff -c -c -r1.10 pgupgrade.sgml
*** doc/src/sgml/pgupgrade.sgml	24 May 2010 17:43:39 -0000	1.10
--- doc/src/sgml/pgupgrade.sgml	25 May 2010 14:50:36 -0000
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*** 16,21 ****
--- 16,31 ----
    9.0.1 -> 9.0.4.
   </para>
  
+  <para>
+   <application>pg_upgrade</> works because, though new features are
+   regularly added to Postgres major releases, the internal data storage
+   format rarely changes.  <application>pg_upgrade</> does its best to
+   make sure the old and new clusters are binary-compatible, e.g.  by
+   checking for compatible compile-time settings.  It is important that
+   any external modules are also binary compatibile, though this cannot
+   be checked by <application>pg_upgrade</>.
+  </para>
+ 
   <sect2>
    <title>Supported Versions</title>
  
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*** 440,446 ****
    <sect2>
    <title>Limitations in migrating <emphasis>from</> PostgreSQL 8.3</title>
   
-  
    <para>
     Upgrading from PostgreSQL 8.3 has additional restrictions not present
     when upgrading from later PostgreSQL releases.  For example,
--- 450,455 ----
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*** 502,509 ****
     For Windows users, note that due to different integer datetimes settings
     used by the one-click installer and the MSI installer, it is only
     possible to upgrade from version 8.3 of the one-click distribution to
!    version 8.4 of the one-click distribution. It is not possible to upgrade
!    from the MSI installer to the one-click installer.
    </para>
   
    <para>  
--- 511,518 ----
     For Windows users, note that due to different integer datetimes settings
     used by the one-click installer and the MSI installer, it is only
     possible to upgrade from version 8.3 of the one-click distribution to
!    version 8.4 or later of the one-click distribution. It is not
!    possible to upgrade from the MSI installer to the one-click installer.
    </para>
   
    <para>  
