Thanks so much for asking! We knew this would be a hot topic. Don't trade in your phone before you get a new phone. We don't want you to be phoneless.
You can accomplish with with pre-orders as well as upgrading in the stores.

Beginning on 9/12, for a limited time, customers can trade-in an iPhone 4, 4s, 5, 5c, and 5s from any carrier with the purchase of a new iPhone 6 or iPhone 6 Plus and receive a $200 gift card (iPhone 4, 4s, 5, 5c) or $300 (iPhone 5s).

This can be done at the store or you can submit for the device trade in online at http://vz.to/17tYyaN.

It is imperative that you disable Find My iPhone http://vz.to/1wwKZbR prior to trading the phone in to us to ensure that you get the proper trade in amount.

The trade-in Apple iPhone must be in good working condition based on Verizon Device Recycling Program requirements:
  • Phone can turn on and off.
  • LCD screen is free of cracks and functioning correctly.
  • Phone and connecting ports are free of visible corrosion and water damage.
  • Battery is present.
  • Phone and charging port are free of cracks and dents, and are not missing any pieces or chunks (not including cords or accessories).
  • For iPhones running iOS 7 or higher, the Find My iPhone feature must be deactivated (turned off) before iPhone is traded in. -Tamara