Here is a reply from this thread:
https://forums.macrumors.com/threads/reserved-att-phone-with-iup-can-use-with-t-mobile.1996588/
"Just came back form Apple Store. I can confirm that with IUP they make you select the carrier that you reserved. I reserved a Verizon I7+ and have T Mobile Service, and when going through the checkout out process, I needed a verizon account. Luckily I was able to snag a T mobile one in store. They wouldnt let me change SIM cards and they had a verizon rep in store stating that specific model has to activate with verizon first and then it can be used for any carrier. Let me know if anyone else had a different outcome."
So looks like a no go if the phone is not on the same account you have with a carrier.