How can I change network carriers by hardware?

Trying to find out what chip or chips on the iPhone 6 logic board store the network carrier lock? I would like to switch to back to AT&T from Sprint. I have a broken AT&T iPhone 6 and a good Sprint iPhone 6 I tried swapping whole logic boards but the AT&T board is damaged So I would like to try just swapping the needed chips. Don't care if S/N's don't match, not going to sell the phone I just want it to function. I understand I run the risk of ending up with two junk logic boards when heating chips for swap, but dont care sprint phone is sitting on a shelve and at the moment is no use to me anyways. Both logic boards are mine and don't have iCloud lock etc. so please save any speeches I'm not doing anything shady. Any help with this issue is appreciated. Thank you