switch carriers

What's the best way to order an iPhone and switching carriers on launch day? Every time I've ordered a new phone it's always been through the same carrier so ordering a new phone was always easy. Should I switch carriers before ordering or on launch night?

I purchased two iPhone 7s when they were announced, but unfortunately bought the T-Mobile version. I was aware of the whole Qualcomm vs Intel modem debate, but I didn't think I would want to switch to Verizon, so unfortunately I pulled the trigger before they had the SIM-free model available...