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I just switched from a 2yr plan to next on an iphone 6 bought via apple.com.
The Apple staff were great!

I walked in the store, told them what I wanted. They processed the return and purchase without any problems. I didn't bring anything with me other than the phone. They replaced the SIM.

Did this at the Century City store.
Did they let you repurchase the same phone that you had already been using?
 
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