Can I Change the Micro Sim To A Nano sim?

I'm buying my mom a new iPhone for Christmas and I noticed the sim cards are different. She has straight talk right now and has a micro sim because she has the iPhone 4s. I was wondering can I buy a new sim card and port all of her stuff over or do I have to buy the activation kit all over again?

Go to the nearest phone shop and request a nano sim

Yes you should be able to request a nanoSIM if need be.
Don't know what's an activation kit though. Usually swapping data between iPhones is just a matter of backing up the old & restoring from that backup on the new using iTunes and/or iCloud.

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