I've done the conversion a couple of times. You'll need a complete. 40 upper which will run $700-800 depending on what parts and extra tools you'll have to buy. You'll need to thin the .45 ejector to be able it run the .40 slide on the .45 frame. You can also replace the ejector. If you like to tinker it's a fun project, but as others have said you'd likely be happier selling the .45 and buying the new 40.