Yes you can do that. I just talked to Apple customer service twice and they said if you use the pay in full option on any carrier then you get an unlocked phone. But of course you need to choose verizon or sprint to get the CDMA + GSM version of the iPhone x unlocked, choose the ATT/T-Mobile version to get GSM only unlocked. Any other payment options are locked to the carrier. I personally am buying the Sprint version of the phone since my friend is letting me use their account details, so that I can activate on my family's Verizon account. (I have some family issues so this is my way of going around the account owner). I'm just gonna pop the SIM card from my iPhone 6 plus into the X and then I'm ready to rock and roll. And for whom it may concern, please be aware, if you go the pay in full route/swap sim in, remember that if you have an insurance plan for you phone through your carrier(like Asurion) you might have to update the coverage to cover the X since its a different phone/more expensive.
Mason M.