If the old phone is a Verizon phone, you can take it into dealer and they will be able to switch everything over for you. The
ESN change (activating your number to his phone) should be free, but they may charge you to transfer your phone book to the new phone.
Another stipulation is how old the phone is. If the phone is a few years old, there is a possibility that it's not 911 compliant. I wouldn't worry about that too much at the moment. The dealer can tell you if the phone can be activated or not when he looks at it.