Q: How can I buy an AT&T iPhone using Upgrade Program and change it to T-mobile?

I am planning to buy the iPhone that will come out this morning, and I heard that the next iPhone is going to run out fast, the configuration that I wanted, like the iPhone 6s and 6s Plus runs out so fast, but this time, I want to start out early because I don't want to miss a day from work, or else I won't get that $5000 per day, so if I don't have an AT&T number, what should I do since I am on T-mobile, and I cannot go to the Apple Store on Friday on time to make a YouTube review and get views, and review it early on my website. I currently am out of AT&T, but I heard that the iPhone is going to be better with Upgrade Program, and I wanted to enroll it. So, why do you need an AT&T number, and why do I have to go to the Apple Store if I want a T-mobile, and last year, I had to wait until October 9 to get my 2nd iPhone 6s Plus from Apple, and one got home from pre-orders unlocked without AppleCare.