I have an iPhone 4 that is jailbroken and unlocked. I will be getting the 4S later this week and plan to pass my 4 to my Dad (who needs to retain jailbreak and unlock. Is it as simple as takings out my sim card, putting in his sim card, plugging the iPhone 4 into his iTunes and choosing restore from back up? Or will that restore to the latest OS and update the modem as well? I'm currently on 4.3.3 and 01.59.00.
Restoring from backup is fine. That will put his stuff on it. Do not press the restore or update button though as that will fully restore the phone which you do not want.