I have two IP4s, one is mine that I upgraded from a 3G. The other is technically my dad's. He was eligible for the upgrade pricing, so we bought it with the intention of unlocking it and selling it. It hasn't been opened yet, it's still sitting in the packaging. Now, to unlock it, it needs to be activated. Since I have an IP4 already, what would happen if I put my SIM card in the other phone? Would it activate normally? I imagine this would be similar to Apple replacing your phone if it has a defect or something, right? AT&T shouldn't care about the IMEI changing, as long as the hardware is the same, right?