IF the phone was purchased as a contract device (substantial discount, or free) then you are obligated to keep the line of service for 24 months - that doesn't mean you have to keep the same device on the line. You just won't get another discounted device or be able to use Verizon's payment plan to get a new device until the 24 months is up. You can buy an iPhone 6 on your own (full price, or find a gently used one on eBay or swappa) and activate that instead of the S3.
