When we've wanted to trade up, we've waited for a buy one, get one free sale, and are just required to re-start the two year contract. No problem. Last time, we actually got four phones on the BOGOF sale, then SirKit was able to get a free one, because his old one broke. Check online for their latest specials, but also check the local store.
They told me I couldn't renew a contract until the first one ended. I have no idea why it's different for me, but apparently it is.