Can you do this?

We have a family plan and have been out of contract and free to leave for years. What we want to do is just cancel our service and sign a new 2 year contract to take advantage of some offers they offer new customers. Like free phones and a better plan. Is this possible or will they say when can only upgrade are phones and plan. (which would be more expensive). If it matters our plan was with Cingular and is not even offered anymore.