I've decided to switch from AT&T to T-Mobile. I ordered a 6S from T-Mobile. I still owe about $300 on my AT&T iPhone 6. I was going to trade it in to T-Mobile and use their reimbursement of my remaining phone payments but my aunt has offered to buy my phone. She is on AT&T and has agreed to pay me the remaining amount. I just need to make sure we do everything right when she takes the phone. She currently has an iPhone 5. So I need to turn off activation lock and Find My iPhone, then reset and erase all data, right? How do I go about removing the iPhone 6 from my iMessage account as well as it being a security device for my iCloud 2nd-factor authentication? Can she just pop her existing SIM card into the iPhone 6 and activate it on her account?