Wish I had seen this before I jumped through all kinds of hoops, over three days, to get a Verizon iPhone X. I originally just wanted to return my T-mobile Space Gray X for a silver one, but all the while I had in my mind the (apparently mythical) superiority of the Qualcomm modem.
Actually it worked out ok, because for 3 days in a row the only 256gb Silver models in my area Apple stores were Verizon, and I was already half-way through the 14-day trial period. So, I wen't to the Verizon store and got a new phone number (using my T-mobile phone), then at 6:00am the next morning, I used the Apple store app to order the Verizon X by opening up a second IUP loan (hence, the need for the new phone number). While picking it up at the Apple store, I returned the T-mobile phone and put the sim card in the new Verizon phone (this required activating with Verizon first). I then called Verizon and converted my post-paid account to prepaid.
The upshot: I now have a "maybe" faster modem, but more importantly, I have a beautiful Silver iPhone X, and get a month to try Verizon in some rural areas where I got "no signal" with T-mobile.