I don't think so, it doesn't ask me to activate my phone when I put in a different carrier SIM like my 6S does. I did get it through the iPhone upgrade program tho. The person at the Apple Store did scan a new ATT sim after scanning the phone tho, maybe this locked it in? (The SIM became active almost instantaneously, and the guy at the Apple Store swapped SIMs in my 6S and threw the old one away so I could keep the 7 Plus sealed for Christmas).
Yup, I shut the phone down before swapping SIMs each time, only cold swap SIMs as I heard hot swapping can confuse the phone or something. My 6S is still under contract, and I'm unable to unlock via ATTs website because I still have one year left with it (traditional 2 year contract, not financing like NEXT). 7 Plus is through iPhone upgrade program, my first time using the program. Also, my 7 Plus is not jail broken, it is on stock 10.1.1 firmware. Unlocked 5S is jail broken on 8.1.3.