Sim Card
I am going to buy a new phone. I want to know if I get a locked phone with AT&T and change the sim card with my sim card in the phone I have now will it work? I have AT&T right now.
iPhone 8 Plus, iOS 14
You would need the right size card.
Learn which size SIM card your iPhone or iPad uses - Apple Support
And if you’re purchasing a 5G iPhone, your old SIM may well not support 5G.
I cannot imagine why you would want to use your old SIM. It contains no Apple-related data.