They are not interchangeable. They are totally different guns. The EZ is hammer fired so it can use the softer recoil spring. The regular Shield is striker fired. Barrels are different. Really nothing is interchangeable between them. I really think S&W made a mistake labeling the EZs as Shields as they are nothing like each other. I have both and they are both nice pistols. I agree that I am not sure the EZ is as reliable as the regular Shield. My wife shoots the EZ and we have had a couple of feeding issues with it. My daughter also has one and it has been back to S&W twice for repairs. It runs fine now but I am not sure I would bet my life on it. She carries a regular Shield.