Are you currently under contract? I would wait until you have completed your contract and are month to month before changing to prepaid. Since the iPhone 4S is a 3G only smartphone it should eligible to activate on a prepaid plan. You should check out the space for more information and questions on prepaid plans including the customer service number for prepaid customers.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.