Verizon is CDMA, but all Verizon iPhones support both CDMA and GSM. Without GSM, a phone is not very useful for roaming since few countries have CDMA networks.
So ... the short answer is that a Verizon iPhone 6 will work perfectly on AT&T (and ships unlocked to boot)!
p.s. When you switch to AT&T, I don't think it is as simple as switching SIM cards. Phone service will work, but I believe that you need to tell AT&T the IMEI number of your phone in order to get data to work.