Not sure about Verizon, but with AT&T you just need to be sure that you call and talk to someone when you order your new iPhone or go into the store and do it that way. Get them to assure you that nothing will change and definitely get their name just in case. This is, of course, if you qualify for an upgrade and it's not in the middle of your contract. They may consider that a reason to remove your unlimited plan.
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