Let’s say you have a 1st Gen / 2.5G (aka EDGE) iPhone, and you’ve upgraded to an iPhone 3G or iPhone 3G S, and you want to update your old (1st Gen) iPhone to iPhone OS 3.0. If you do, you’re old iPhone will need to be activated. It might not be obvious, but all you need to do is
- pop the SIM out of your activated iPhone 3G (S) and pop it into the old iPhone.
- Connect the old iPhone to a machine with iTunes, and it will activate.
- After that, you can take the SIM out of the old iPhone, and put it back into your current iPhone, and will is well. At least until the next iPhone OS update.