I have 4 phones on my account. 2 are out of contract, 2 are not. I plan on buying two iPhone 5s on launch day. 1 will replace an off contract phone, and the other will replace a phone currently on contract. However, I plan on purchasing both iPhones using the two off-contract phones upgrade option. After they arrive in the mail, what do I need to do to swap the new iPhone for one still on contract? Anyone done this recently and have any quick tips and/or pointers to make it happen smoothly?