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Just go to the AT&T store and have them scan the QR code in settings->cellular -> add cellular plan

Make sure wifi is enabled.
I had to do this yesterday.
 
If I’m trading in my iPhone 12 can I just take the sim out and stick it in my 13 pro max in store, avoid the activation charge with ATT (not financing through att or selecting any particular carrier)
 
If I’m trading in my iPhone 12 can I just take the sim out and stick it in my 13 pro max in store, avoid the activation charge with ATT (not financing through att or selecting any particular carrier)

Yes for the sim. Not sure about the bs activation charge! I buy straight from Apple and swap my sim in the car. Done! No activation fuss/charge ( TMobile )
 
Everyone's results must vary - I've had an iPhone X, 11 Pro, 12 Pro and now the 13 Pro. I have all of the boxes and each of them did have a SIM tool included. The 11, 12 and 13 Pro's were purchased as officially unlocked devices. I can't recall how we purchased the X, I suppose I could look at the model # on the box but isn't important...lol.
 
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