After my 3G iPhone was replaced, the replacement was activated with my SIM. It was great to have my phone back, but at the same time my daughter needs hers back. The 2G iPhone was stuck on the Different SIM detected screen, and wouldn't allow me to pass authentication without completely messing up my phone SIM. After half a day of restoring the phone to the 3.1.3 firmware, then shift+restoring the phone to the custom firmware, then re-restoring to the 3.1.3 firmware, I almost gave up. The phone would require authentication. Then I found iREB. iREB puts the phone into a restore mode that allows for bypassing of the 16xx errors that I had been receiving (both 1601 and 1604). However, it wouldn't get to the white screen as it was supposed to. Following this post on iH8sn0w's forums, I downloaded the 3.1.2 firmware, ran redsn0w up to the point of the white screen, disconnected, and did a shift+restore with the custom 3.1.3 firmware sn0wbreeze created for me. That finished out the update with a jailbreak, activation, and a free and clear SIM that is not connected to any network. Sweet!