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If you subscribe to a new topic using the same callback and store the result in the same Subscriber object, storing that new value in the original variable should unsubscribe the other topic.

EXAMPLE: untested, probably buggy

class Reader
{
public:
  Reader(ros::NodeHandle node)
  {
    node_ = node;
    // subscribe to original topic
    topic_ = node.subscribe("first_topic", 10,
                            &Reader::callback, (Reader *) this);
  }

private:
  void callback(const std_msgs::String::ConstPtr &msg)
  {
    if (msg->data == "switch")
      {
        // subscribe to other topic
        topic_ = node_.subscribe("next_topic", 10,
                                 &Reader::callback, (Reader *) this);
      }
    else
      {
        // log message from current topic
        ROS_INFO_STREAM(msg->data);
      }
  }

  // class variables
  ros::NodeHandle node_;
  ros::Subscriber topic_;           // current topic subscription
};

int main(int argc, char **argv)
{
  ros::init(argc, argv, "your_node");
  ros::NodeHandle node;
  Reader read(node);
  ros::spin();
  return 0;
}