Changing BYOD phones for new BYOD phones

I signed up the family with 4 iPhones bought sim unlocked from Apple. 

I have have bought new Sim unlocked phones from Apple that I want to use on my Xfinity mobile phone lines. How can I change the devices on my account?

I tried chatting with someone and they said I had to port out and back again, that’s sounds incredulous. 

The sims work in the new phones, but I want to give my old phones to parents  for Xfinity mobile and the setup complains that the phone is in use by someone else (me!)