I have two iPhone's on my AT&T account. One is an iPhone 6 (under contract) and the other an iPhone 5 (out of contract). I just ordered a new IPhone 6 Plus upgrading the iPhone 5. When the 6 Plus arrives and is activated, can I swap SIM's between the 6 and 6 Plus? I use the 6 and my wife the 5. I want to start using the 6 Plus and let her use my old 6.