Hi, I have an iPhone 5 with AT&T grandfathered unlimited. I currently pay $63/month from my last 2-year contract, which expired around January of this year (2015). I live in California. Question: I want to buy an unlocked AT&T iPhone 6s. Will I need a new SIM card, or do I just put my current iPhone 5 phone's SIM card into the new 6s? Thanks.