How long the route is from Changsha (regular train) to Changsha nan (speed train)?

Hello,

I need some advice. I have a train going from Zhangjiajie to Changsha by regular train and then transfer to the high speed train from Changsha nan to Tangjiawan. How long will it take to get from regular train to high speed one by Changsha metro? My window to get there is less than a hour so I'm a bit nervous.

Many thanks!