can't understand the structural change in tree while converting a bst to max heap

I was looking at the code challenge from gfg BST to max heap:

Given a Binary Search Tree. Convert a given BST into a Special Max Heap with the condition that all the values in the left subtree of a node should be less than all the values in the right subtree of the node. This condition is applied on all the nodes in the so converted Max Heap.

I could not solve the challenge.

So I had a look at the following solution code:

class Solution
{
    public static ArrayList<Integer> list=new ArrayList<>();
    static int i=0;

    public static void InOrderTraversal(Node root){       
        if(root == null) return ;
        InOrderTraversal(root.left);
        list.add(root.data);
        InOrderTraversal(root.right);
    }

    public static void PostOrderTraversal(Node root){
        if(root == null )return; 
        PostOrderTraversal(root.left);
        PostOrderTraversal(root.right);
        root.data= list.get(i);
        i++;
    }

    public static void convertToMaxHeapUtil(Node root)
    {
        InOrderTraversal(root);
        PostOrderTraversal(root);
    }
}

I don't see how this code could change the structure of the tree, as it neither adds nor deletes any nodes. Here is the second sample testcase copied from gfg official website:

Input BST:

             3
           /   \
          1     5
           \   /  \
            2 4    6
                    \
                     7

Expected Max heap output:

           7
         /   \
        3     6
      /   \  /   \
     1    2 4     5

I can't understand how it is working on that test case.

Driver code for the problem:

//{ Driver Code Starts
//Initial Template for Java

import java.util.LinkedList; 
import java.util.Queue; 
import java.io.*;
import java.util.*;

class Node{
    int data;
    Node left;
    Node right;
    Node(int data){
        this.data = data;
        left=null;
        right=null;
    }
}

class Tree {
    
    static Node buildTree(String str){
        
        if(str.length()==0 || str.charAt(0)=='N'){
            return null;
        }
        
        String ip[] = str.split(" ");
        // Create the root of the tree
        Node root = new Node(Integer.parseInt(ip[0]));
        // Push the root to the queue
        
        Queue<Node> queue = new LinkedList<>(); 
        
        queue.add(root);
        // Starting from the second element
        
        int i = 1;
        while(queue.size()>0 && i < ip.length) {
            
            // Get and remove the front of the queue
            Node currNode = queue.peek();
            queue.remove();
                
            // Get the current node's value from the string
            String currVal = ip[i];
                
            // If the left child is not null
            if(!currVal.equals("N")) {
                    
                // Create the left child for the current node
                currNode.left = new Node(Integer.parseInt(currVal));
                // Push it to the queue
                queue.add(currNode.left);
            }
                
            // For the right child
            i++;
            if(i >= ip.length)
                break;
                
            currVal = ip[i];
                
            // If the right child is not null
            if(!currVal.equals("N")) {
                    
                // Create the right child for the current node
                currNode.right = new Node(Integer.parseInt(currVal));
                    
                // Push it to the queue
                queue.add(currNode.right);
            }
            i++;
        }
        
        return root;
    }
    static void postOrder(Node root)
    {
        if(root == null)
            return;
            
        postOrder(root.left);
         postOrder(root.right);
        System.out.print(root.data+" ");
    
    }
    
    public static void main (String[] args) throws IOException{
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            
            int t=Integer.parseInt(br.readLine());
    
            while(t > 0){
                String s = br.readLine();
                Node root = buildTree(s);
                
                Solution g = new Solution();
                g.convertToMaxHeapUtil(root);
                postOrder(root);
                
                System.out.println();
                    
                t--;
            
        }
    }
}
// } Driver Code Ends