I would like to upgrade my 6+ to a 6s+. I have a 2 year contract with AT&T however I am not yet eligible for a full upgrade. My girlfriend has a separate AT&T contract and is eligible for a full upgrade. She currently has an iPhone 5 and said she would be happy to have my old 6+. She's planning to buy the 6s+ for the subsidized price and give it to me in exchange for my 6+. When the new phone arrives, would it be as simple as putting the SIM card from her iPhone 5 into my old 6+ and me putting my SIM card into the new 6s+?