=== modified file 'account/invoice.py'
--- account/invoice.py	2010-10-27 12:59:05 +0000
+++ account/invoice.py	2010-11-09 19:12:22 +0000
@@ -596,6 +596,7 @@
         self.write(cr, uid, ids, {'state':'draft'})
         wf_service = netsvc.LocalService("workflow")
         for inv_id in ids:
+            wf_service.trg_delete(uid, 'account.invoice', inv_id, cr)
             wf_service.trg_create(uid, 'account.invoice', inv_id, cr)
         return True