I am the person that will be helping you today.
I am sorry to report there is no way to use your At&t phone with Verizon. The reason being is that the two companies use different types of cell networks. You can however, take a look at Verizon's trade in program, which may give you a discount on a new phone for your old phone. The following page will give more information regarding the trade in program: http://www.verizonwireless.com/landingpages/device-trade-in/
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Thank you again, - Jason