There are buses from Changsha to Shanggao directly, you could take it at the local bus station.

Thank you. That is good to know. Which bus station in Changsha is the origin of the Shanggao bus, how long does it take, and what is the approximate cost? I assume there is also the same bus in the return direction as well (Shanggao to Changsha)?

You can take bus to Shanggao Direction at the Changsha East Bus Station(1021, Yuandayi Road, Furong District). If there are buses to Shanggao, take it. If not, you can take bus from Changsha to Yichun, after arriving, you can take the Yichun-Shanggao Bus to Shanggao.