diff -Naur moodle197a/grade/edit/tree/index.php moodle197b/grade/edit/tree/index.php
--- moodle197a/grade/edit/tree/index.php	2009-07-07 12:46:50.000000000 -0500
+++ moodle197b/grade/edit/tree/index.php	2009-12-23 10:42:55.000000000 -0600
@@ -84,6 +84,15 @@
         error('Incorrect category id!');
     }
     $data->aggregation = $aggregationtype;
+    if ($grade_category->aggregation == GRADE_AGGREGATE_WEIGHTED_MEAN AND $aggregationtype <> GRADE_AGGREGATE_WEIGHTED_MEAN) {
+    	// GO THROUGH AND SET THE AGGREGATIONCOEF FOR ALL CHILDREN TO ZERO
+    	$grade_category->children = $grade_category->get_children();
+    	foreach ( $grade_category->children as $useditem) {
+    		$useditem['object']->grade_item = $useditem['object']->get_grade_item();
+    		$useditem['object']->grade_item->aggregationcoef = 0;
+    		$useditem['object']->grade_item->update();
+    	}
+    }    
     grade_category::set_properties($grade_category, $data);
     $grade_category->update();
     grade_regrade_final_grades($courseid);