Index: lib/grade/grade_category.php
===================================================================
RCS file: /cvsroot/moodle/moodle/lib/grade/grade_category.php,v
retrieving revision 1.96.2.30
diff -u -r1.96.2.30 grade_category.php
--- lib/grade/grade_category.php	29 Apr 2009 13:44:18 -0000	1.96.2.30
+++ lib/grade/grade_category.php	28 Jan 2010 07:02:05 -0000
@@ -209,13 +209,18 @@
 
         $this->apply_forced_settings();
 
-		// these are exclusive
+        // these are exclusive
         if ($this->droplow > 0) {
             $this->keephigh = 0;
         } else if ($this->keephigh > 0) {
             $this->droplow = 0;
         }
 
+        // If we're switching away from "Sum of grades" mode we should set the maxgrade for this
+        // category's grade item back to whatever it was before we entered "Sum of grades" in the
+        // first place.
+        $this->restore_non_sum_grademax();
+
         // Recalculate grades if needed
         if ($this->qualifies_for_regrading()) {
             $this->force_regrading();
@@ -1423,5 +1428,49 @@
         $sql = "UPDATE {$CFG->prefix}grade_items SET needsupdate=1 WHERE itemtype='course' or itemtype='category'";
         execute_sql($sql, false);
     }
+
+    /**
+     * Intended to be called while updating the category. Checks to see whether the category is
+     * switching away from "Sum of grades" aggregation, and if so, restores the maxgrade for the
+     * category's gradeitem back to what it was before the category entered "Sum of grades" mode in
+     * the first place.
+     *
+     * This is because "Sum of grades" calculates the field completely differently from all other
+     * aggregation modes (in which it's an arbitrarily editable text field), so the user could
+     * otherwise lose information and be confused when switching between "Sum of grades" and other
+     * grade aggregation methods. (MDL-21449)
+     *
+     * @return void
+     */
+    function restore_non_sum_grademax() {
+        global $CFG;
+        if ($this->aggregation != GRADE_AGGREGATE_SUM) {
+            $catprevstate = grade_category::fetch(array('id'=>$this->id));
+            if ($catprevstate->aggregation == GRADE_AGGREGATE_SUM ) {
+                $gradeitem = $this->load_grade_item();
+
+                // The last history entry before the current "Sum of grades" setting was entered
+                $lastnonsumtime = get_field_select('grade_categories_history', 
+                        'max(timemodified)', 'oldid = ' . $this->id . ' and aggregation <> 13');
+
+                // When the current "Sum of grades" setting actually started
+                $earliestsumtime = get_field_select('grade_categories_history',
+                        'min(timemodified)', 'oldid = ' . $this->id . ' and aggregation = 13 and '
+                        . 'timemodified >= ' . $lastnonsumtime );
+
+                // The grademax for the last grade_item entry before the "Sum of grades" started
+                $prevmaxgradearray = get_records_select('grade_items_history', 'timemodified < '
+                        . $earliestsumtime . ' and oldid = ' . $gradeitem->id,
+                        'timemodified desc', 'grademax', 0, 1);
+                if (!$prevmaxgradearray || count($prevmaxgradearray) == 0) {
+                    $gradeitem->grademax = 100;
+                } else {
+                    $prevmaxgrade = current($prevmaxgradearray);
+                    $gradeitem->grademax = $prevmaxgrade->grademax;
+                }
+                $gradeitem->update();
+            }
+        }
+    }
 }
 ?>