I would suggest calling the prepaid customer service number at 1-888-294-6804 for the steps on how to transfer the phone from a postpaid account to a prepaid account.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
You can, but if you are on prepaid, you'll want to activate the old phone at the end of the month to avoid having to pay for another month of service. Each device change requires a new payment...even if you are already paid ahead.