Can you please go with me to the Seattle or Bellevue Apple Store and help me make this transaction? I went to both over the weekend and things did not go well. They didn't seem to understand the basic process you outlined above.
Seriously though I showed up at Apple with a credit card and a SIM card 48 hours ago and they were not able to sell me a phone. To make it even weirder, I had the (unopened) phone I wanted in a bag, but it is a "locked" one from TMobile. I thought that was ancient history.
One day Apple will want my money!
In fact, I did this two years ago with the 6 Plus. I bought a T-Mobile phone from Apple because they were unlocked (at the time the others weren't, even if purchased directly from Apple) and used it with my AT&T SIM.