Manipulate tag tree from tagged pdf

Question

Question: I was working on tags for accessibility with iText7 and I want to manipulate the structure of the tag tree.

The task would be like. First of all, extract the tag tree from tree. Then, move the node to it's parent node.

The demonstrate structure to be like this.

[Before]
<H1>
    <H2>
        <Span>
              "Description"
<H3>
    <Span>
          "Item"

[After]
<H1>
    <Span>
          "Description"
<H2>
    <Span>
          "Description"
<H3>
    <Span>
          "Item"

Is it possible to use some libraries to approach my goal? As my understanding, the tagUtils in iText might be helpful. I already trying parse tagged pdf with iText under pdf specification. Yet, it's extremely Unintuitive. Since the well-known pdf tools, Acrobat can show the tags on the panel to make the pdf be accessibility. It must be some libraries to extract the tag tree from tagged-pdf without pain.

Imgur

Any response would be extremely helpful. Thanks.