When you bought the AT&T phone, did you swap out the SIM with your active SIM or did you try to activate the SIM inside the phone? If you did the latter, was there a fee involved (even if you were able to later call and get it waived)?
That's the beauty part--at least with sticking with the same mobile carrier--I did the latter. Put an active SIM from my X into a fully-paid but unlocked AT&T XR. Didn't have to do anything. No fees. Every time I've done this (meaning, paying full price for either an AT&T or a "SIM-Free" iPhone), I've never seen any activation fees when moving an active SIM to a new "paid full price" iPhone.