StructuredStorageReader reader = new StructuredStorageReader(docPath);

WordDocument doc = new WordDocument(reader);

WordprocessingDocument docx = WordprocessingDocument.Create(docxPath, DocumentType.Document);

public static void Convert(WordDocument doc, WordprocessingDocument docx)

Converter.Convert(doc, docx);

using b2xtranslator.StructuredStorage.Reader;
using b2xtranslator.DocFileFormat;
using b2xtranslator.OpenXmlLib.WordprocessingML;
using b2xtranslator.WordprocessingMLMapping;
using static b2xtranslator.OpenXmlLib.OpenXmlPackage;
namespace ConverterToXml.Converters
{
    public class DocToDocx
    {
        public void ConvertToDocx(string docPath, string docxPath)
        {
            StructuredStorageReader reader = new StructuredStorageReader(docPath);
            WordDocument doc = new WordDocument(reader);
            WordprocessingDocument docx = WordprocessingDocument.Create(docxPath, DocumentType.Document);
            Converter.Convert(doc, docx);
        }
    }
}

public virtual byte[] CloseWithoutSavingFile()
{
    var writer = new OpenXmlWriter();
    MemoryStream stream = new MemoryStream();
    writer.Open(stream);
    this.WritePackage(writer);
    writer.Close();
    byte[] docxStreamArray = stream.ToArray();
    return docxStreamArray;
}

public virtual void Close()
{
     // serialize the package on closing
    var writer = new OpenXmlWriter();
    writer.Open(this.FileName);
    this.WritePackage(writer);
    writer.Close();
}

public MemoryStream ConvertToDocxMemoryStream(Stream stream)
{
    StructuredStorageReader reader = new StructuredStorageReader(stream);
    WordDocument doc = new WordDocument(reader);
    var docx = WordprocessingDocument.Create("docx", DocumentType.Document);
    Converter.Convert(doc, docx);
    return new MemoryStream(docx.CloseWithoutSavingFile());
}

1.1 Первый.Первый
1.2 Первый.Второй
1.2.1   Первый.Второй.Первый
1.2.2   Первый.Второй.Второй
Какая-то строчка
1.2.3   Первый.Второй.Третий
2.  Второй
2.1 Второй.Первый

xmlns:wp14="http://schemas.microsoft.com/office/word/2010/wordprocessingDrawing"

public const string WordprocessingML2010 = "http://schemas.microsoft.com/office/word/2010/wordml";

this._writer.WriteAttributeString("xmlns", "w14", null, OpenXmlNamespaces.WordprocessingML2010);

this._writer.WriteAttributeString("w14", "paraId", OpenXmlNamespaces.WordprocessingML2010, Guid.NewGuid().ToString());
            this._writer.WriteAttributeString("w14", "textId", OpenXmlNamespaces.WordprocessingML2010, "77777777");