If you are switching from another Verizon phone that has a nano-SIM (e.g. iPhone 5 or 5s), just have the Apple rep give you the phone without unboxing it. Take your SIM out and put it in the new phone before turning it on.
If you are upgrading from an iPhone 4 or 4s, which has a micro-SIM, or an Android that does not have a nano-SIM, just make sure the Apple rep has scanned the bar code and "activated" the line. When you turn on the phone for the first time, it will activate the new SIM card.
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Appreciate the feedback. The Sales Rep at the Verizon store told me if I purchased the IPhone 6 at the Apple Store that I should not let them activate it. I should bring it to the Verizon store unopened. He stated it took them (Verizon) a half an hour to undo what the Apple Store would do and set up the phone. There is BS someplace. I'm smelling the Verizon Sales Rep on this one.
There is one case when you absolutely should not let the Apple Store rep open the box and activate the phone. That's if you are trying to preserve an unlimited data plan while still getting the subsidized price. It's called the "Best Buy" method from where it was first discovered to work.
That involves ordering the upgrade, but then taking the new phone out of the box, KEEPING IT TURNED OFF, removing the SIM and throwing it away, and then inserting your original SIM from your old phone and THEN turning it on and letting it activate. Supposedly, as long as that new SIM never gets activated (which is why people recommend throwing it away), you will not lose your unlimited data plan. But if you let the rep (or anyone else) turn on the phone with the new SIM in it, it will update your account and convert your unlimited data into a mobile share plan with limited data.