Porting from another carrier and changing phone numbers on the same carrier are 2 entirely different things.
Porting is taking your phone number from Carrier A, say Sprint, and moving it to Carrier B, say AT&T. If you're still in a contract with Sprint, when you bring your number over, you will be stuck with the early termination fee. And yes, the 2 year contract is bound to the phone number (well, kinda. It's bound to the phone line, which is bound to the number. It's like if A=B and B=C, A=C) but when they say it's not bound to the phone itself, that means you're free to buy new phones and swap them all you want and you don't have to renew your contract or pay any early termination fees (but unless you renew, you'll be paying full price for the phone, but that's another issue).
What the OP wants to do is change the phone number associated with his AT&T account. He's not porting, or canceling or anything like that. He just has a phone number and for whatever reason, wants to change it. That has absolutely nothing to do with contracts and you can change your number if you're in contract. There's a fee for it, I want to say its 36 bucks, but don't quote me on that. That's a number change fee and applies regardless of whether or not you're under contract. Has nothing to do with the ETF.