UIApplication uiapp = commandData.Application;
uiapp.OpenAndActivateDocument("C:\\Temp\\Test.rfa");
UIDocument uidoc = uiapp.ActiveUIDocument;
Application app = uiapp.Application;
Document doc = uidoc.Document;
FamilyManager fm = doc.FamilyManager;
FamilyParameter param =  fm.get_Parameter("ReplacementPrmtr");
            if (null != param)
            using (Transaction tr = new Transaction(doc, "Renaming")) {
                tr.Start();
                fm.RenameParameter(param, "TEST_SUCCESS");
                tr.Commit();
            }