diff --git a/subversion/include/svn_error_codes.h b/subversion/include/svn_error_codes.h
index b25052b..f7895bc 100644
--- a/subversion/include/svn_error_codes.h
+++ b/subversion/include/svn_error_codes.h
@@ -345,6 +345,10 @@
               SVN_ERR_WC_CATEGORY_START + 19,
               "Invalid operation on the current working directory")  
 
+  SVN_ERRDEF (SVN_ERR_WC_FORMAT_UPGRADE,
+              SVN_ERR_WC_CATEGORY_START + 20,
+              "Unable to upgrade working administrative format")  
+
   /* fs errors */
 
   SVN_ERRDEF (SVN_ERR_FS_GENERAL,
diff --git a/subversion/libsvn_wc/lock.c b/subversion/libsvn_wc/lock.c
index 608865b..841dd5f 100644
--- a/subversion/libsvn_wc/lock.c
+++ b/subversion/libsvn_wc/lock.c
@@ -85,14 +85,44 @@
 do_close (svn_wc_adm_access_t *adm_access, svn_boolean_t preserve_lock);
 
 
+/* Maybe upgrade the working copy directory represented by ADM_ACCESS
+   to the latest 'SVN_WC__VERSION'.  This may involve renaming some
+   administrative files, therefore ADM_ACCESS must contain a write
+   lock.  Use POOL for all temporary allocation.
+
+   Not all upgrade paths are necessarily supported.  For example,
+   upgrading a version 1 working copy results in an error.
+
+   Sometimes the format file can contain "0" while the administrative
+   directory is being constructed; calling this on a format 0 working
+   copy has no effect and returns no error. */
 static svn_error_t *
-upgrade_format (svn_wc_adm_access_t *adm_access, apr_pool_t *pool)
+maybe_upgrade_format (svn_wc_adm_access_t *adm_access, apr_pool_t *pool)
 {
-  /* ### We'll write the upgrade code later.  For now, punt. */
-  return svn_error_createf
-    (SVN_ERR_WC_CORRUPT, NULL,
-     "TODO: '%s' needs a format upgrade, but we haven't written the code yet",
-     adm_access->path);
+  if (adm_access->wc_format == 1)
+    {
+      return svn_error_createf
+        (SVN_ERR_WC_FORMAT_UPGRADE, NULL,
+         "working copy format 1 is too old to upgrade, in '%s';\n"
+         "please check out your working copy again",
+         adm_access->path);
+    }
+  else if (adm_access->wc_format == 2)
+    {
+      const char *path = svn_wc__adm_path (adm_access->path, FALSE, pool,
+                                           SVN_WC__ADM_FORMAT, NULL);
+
+      SVN_ERR (svn_io_write_version_file (path, SVN_WC__VERSION, pool));
+      adm_access->wc_format = SVN_WC__VERSION;
+    }
+  else if (adm_access->wc_format > SVN_WC__VERSION)
+    {
+      return svn_error_createf
+        (SVN_ERR_WC_FORMAT_UPGRADE, NULL,
+         "this client is much too old to work with working copy '%s';\n"
+         "please get a newer Subversion client",
+         adm_access->path);
+    }
 
   return SVN_NO_ERROR;
 }
@@ -100,7 +130,12 @@
 
 /* Create a physical lock file in the admin directory for ADM_ACCESS. Wait
    up to WAIT_FOR seconds if the lock already exists retrying every
-   second. */
+   second. 
+
+   Note: most callers of this function determine the wc_format for the
+   lock soon afterwards.  We recommend calling maybe_upgrade_format()
+   as soon as you have the wc_format for a lock, since that's a good
+   opportunity to drag old working directories into the modern era. */
 static svn_error_t *
 create_lock (svn_wc_adm_access_t *adm_access, int wait_for, apr_pool_t *pool)
 {
@@ -124,17 +159,7 @@
             return err;
         }
       else
-        {
-          /* We have a write lock.  If the working copy has an old
-             format, this is the time to upgrade it. */
-          if ((adm_access->wc_format > 0)
-              && (adm_access->wc_format < SVN_WC__VERSION))
-            {
-              SVN_ERR (upgrade_format (adm_access, pool));
-            }
-          
-          return SVN_NO_ERROR;
-        }
+        return SVN_NO_ERROR;
     }
 
   return svn_error_createf (SVN_ERR_WC_LOCKED, NULL,
@@ -279,8 +304,11 @@
       apr_hash_set (lock->set, lock->path, APR_HASH_KEY_STRING, lock);
     }
 
+  /* We have a write lock.  If the working copy has an old
+     format, this is the time to upgrade it. */
   SVN_ERR (svn_wc_check_wc (path, &lock->wc_format, pool));
-
+  SVN_ERR (maybe_upgrade_format (lock, pool));
+  
   lock->lock_exists = TRUE;
   *adm_access = lock;
   return SVN_NO_ERROR;
@@ -350,8 +378,12 @@
     {
       lock = adm_access_alloc (svn_wc__adm_access_unlocked, path, pool);
     }
+
   if (! under_construction)
-    lock->wc_format = wc_format;
+    {
+      lock->wc_format = wc_format;
+      SVN_ERR (maybe_upgrade_format (lock, pool));
+    }
 
   if (tree_lock)
     {