Using a Spring Iphone 8+ on At&t?

I have an iphone 8 plus, but it's on Sprint. I want to use it on At&t. Can I just swap sim cards? Or do I have to go to sprint and get it unlocked then swap sim cards?

First you need to get Sprint to unlock the phone. Once it is unlocked, it should theoretically work. Sprint is kind of weird when it comes to unlocking phones so they may be hesitant.

You'll need your Sprint account PIN in order for them to unlock your phone.

I think Master of Puppets is right and that unlocked iPhones 6s, SE, and newer should work with both Verizon/Sprint style system and AT&T/T-Mobile system