diff --git a/addons/product/product.py b/addons/product/product.py
index 10dae7bb253..76b016f2008 100644
--- a/addons/product/product.py
+++ b/addons/product/product.py
@@ -728,12 +728,13 @@ class product_template(osv.osv):
         ''' Store the standard price change in order to be able to retrieve the cost of a product template for a given date'''
         if isinstance(ids, (int, long)):
             ids = [ids]
-        if 'uom_po_id' in vals:
-            new_uom = self.pool.get('product.uom').browse(cr, uid, vals['uom_po_id'], context=context)
+        if 'uom_id' in vals:
+            new_uom = self.pool.get('product.uom').browse(cr, uid, vals['uom_id'], context=context)
             for product in self.browse(cr, uid, ids, context=context):
-                old_uom = product.uom_po_id
-                if old_uom.category_id.id != new_uom.category_id.id:
-                    raise osv.except_osv(_('Unit of Measure categories Mismatch!'), _("New Unit of Measure '%s' must belong to same Unit of Measure category '%s' as of old Unit of Measure '%s'. If you need to change the unit of measure, you may deactivate this product from the 'Procurements' tab and create a new one.") % (new_uom.name, old_uom.category_id.name, old_uom.name,))
+                old_uom = product.uom_id
+                if old_uom != new_uom:
+                    if self.pool.get('stock.move').search(cr, uid, [('product_id', 'in', [x.id for x in product.product_variant_ids]), ('state', '!=', 'cancel')], context=context):
+                        raise osv.except_osv(_('Unit of Measure can not be changed anymore!'), _("As there are existing stock moves of this product, you can not change the Unit of Measurement anymore. "))
         if 'standard_price' in vals:
             for prod_template_id in ids:
                 self._set_standard_price(cr, uid, prod_template_id, vals['standard_price'], context=context)
@@ -785,7 +786,7 @@ class product_template(osv.osv):
         return True
 
     _constraints = [
-        (_check_uom, 'Error: The default Unit of Measure and the purchase Unit of Measure must be in the same category.', ['uom_id']),
+        (_check_uom, 'Error: The default Unit of Measure and the purchase Unit of Measure must be in the same category.', ['uom_id', 'uom_po_id']),
     ]
 
     def name_get(self, cr, user, ids, context=None):