Original 3g SIM tray has IMEI and Serial #'s printed on it. OEM will have a made up IMEI and Serial # printed on it, but not yours or anybody else. I once asked a Apple Genius about this and he said it takes more than a week to get a new SIM tray from Apple, and I said why? Genius said because they print your IMEI and Serial # on the tray. Don't know if this true. Regarding case swap, Yes you can change the outer body of your iPhone, but even in experienced hands it does not always go perfect. If first time, good luck with it.