Could u carry it for self defense (yes). Would i recommend it (NO). You are protecting your life and your loved ones. Dont risk the possible failure of a converted pistol. Use the .40 for self defense and use the 9mm for cheaper range time.

Going from 9mm to .40 is possible (at least with Shield). You must grind a tiny bit of the .4o barrel (tangs) down to make it fit into the 9mm slide. Since the 9mm slide is smaller here, u have to mod the .40 barrel or it will not fit. It's been done and a few members here are still running the conversion from 9mm to .40.