Actually all Verizon LTE phones use SIM cards. Their LTE network isn't CDMA so it requires a SIM. Any Verizon iPhone 6 can also work on any network. My mom is currently using a Verizon 6 Plus on AT&T and she just had to pop her AT&T SIM card in and it worked with no problems.
If u buy a full price iatt/tmobile/Verizon Phone 6 from the apple store it's gsm unlocked. A Verizon 6 is gsm unlocked regardless if under contract or not. That's it
If u look in the iPhone forums there's multiple threads discussing this.