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();
}