What happens when you switch the SIM cards in an iPhone 4 and an iPhone 4s?

I want to use my moms old iPhone 4 because my old iPhone 4s is pretty beat up and hers is in better condition. She has recently switched to an iPhone 5c, and does not use her iPhone 4. I've heard if you just switch SIM cards that is how you can switch phones. Since hers is no longer connected to a number does that affect anything? If the SIM card holds the number and all that, does it carry over apps music photos etc? I'm really asking how to just basically take my phones data and put it into an unused phone. Is it hard to do and how do you do it? Is the SIM card the main thing to switch? Oh and also we're on different apple accounts if that affects it at all.

Switching the sim card is the main thing. It will transfer your phone number, and other contacts, if you saved your contacts onto your sim card (not on the phone). Your apps, music, video, photos, notes, etc. Will not be transferred. You would need to either re-download the apps again on your mom's phone, or back up your iPhone to iTunes, and then sync your moms phone and choosing your phones backup in iTunes to sync to your moms phone. So in a nutshell, yes switch your sim card over. That will allow the iPhone 4 to receive your calls, and texts. Also check the apple account into he settings> app store. And make sure to sign in to your account.

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