Transfer Upgrade Eligibility but I still have installments left

I have an iPhone 12 Pro and still have installments left on my plan. My husband has an iPhone XS and is upgrade eligible. I want the iPhone 13 Pro, so I upgraded my husbands line and got the iPhone 13 Pro.  He is okay with switching phones so that I can use the iPhone 13 Pro and he will use the iPhone 12 Pro.  The new iPhone 13 Pro came with a new SIM card. How do we switch phones? i.e. do I turn on the new iPhone 13 Pro and put in the new SIM card - but would this be tied to his phone number and would make his phone obsolete? Do I need to call AT&T to reassign SIM cards to the correct phone number? Please help!