Change my iPhone from GSM to CDMA

With a few exceptions, most iPhones made for the past several years have both already. The iPhone 7 is an exception to that as the AT&T and T-Mobile devices lacked the CDMA modem.

But for most, to change carriers, your iPhone must not be carrier locked. If not, all you need is a SIM for the carrier you want to switch to. With the switch to LTE, the whole distinction of GSM and CDMA is really not even applicable anymore to recent models and carrier service.

GSM and CDMA are physical hardware in the phone. You cannot change them.

~Lyssa

Keep in mind that even if your phone contains CDMA hardware, the carriers are not obligated to activate it on their network if it was purchased for use on another network.

What carrier were you on previously and what CDMA carrier are you wanting to switch to?