Since LTE requires sim cards, could you buy an ATT iPhone, unlock it, and make it verizon 4G? Please no nasty comments because I am younger than you think.......
I wont say it isn't possible (though I'm pretty sure it's not) but if it were possible, why would you want to buy a 1-2 year old piece of hardware (depending on when they roll out the new LTE)? No doubt, Verizon will be releasing an LTE device by next summer. Even more than likely is that Apple will make a new LTE capable device that is universal for AT&T and Verizon. So...pretty sure it would be pointless to do. Plus, you have to take into consideration that Apple will also have to do another antenna change for whatever model comes out next and the current AT&T model will not be compatible with LTE signals. Theres a number of reason why this probably will not work but even if it did it would be a very bad idea. Just my .02 though.
My guess is that in the future when the iPhone supports LTE you could technically swap sims between the two networks, however the phones will likely be carrier locked as they are today.