Switch between iPhone 4 and iPhone 4S I have an iPhone 4 and my significant other has an iPhone 3G. They just bought an iPhone 4S and is willing to trade it for my iPhone 4. My phone is already registered under my name and all, while the new iPhone 4S isn't registered under anyone's name yet. What is the best way to go about transferring all our data correctly? Do we need to go to our cell carrier or to Apple?
Back up both phones in iTunes. You can do this on separate computers to avoid confusion.
Factory reset both iPhones.
Insert the new SIMs in the phones.
Restore the phones from the backups you made in step 1. Specifically, restore the 4S from the 4's backup and the 4 from the 3G's backup.


If it doesn't work out you can just restore your phone from the latest backup.