I don't think notty is going to lower the gravity of this beer at all. Westmalle yeast is much more attenuative provided you give it the right temperature and time. Theres a pretty good chance you are already near terminal gravity. Take a gravity reading and see before you even consider doing anything else.
I really, really wouldn't consider moving this beer onto another yeast, especially notty. Give it some time and it will take care of your beer.