How to create a .docx file from another .docx file using the Open XML SDK?

Question

C++ / C#

There is a DOCX file with a form. You need to make a copy of it, but at the same time make a replacement in the text of the form (replace Number with a number, and replace DateS with the current date).
I do it like this:

string fileSrc = Server.MapPath("/files/source_pko.docx");
string fileRes = Server.MapPath("/files/pko000.docx");
using (var docSrc = WordprocessingDocument.Open(fileSrc, false))
using (var docRes = WordprocessingDocument
  .Create(fileRes, WordprocessingDocumentType.Document))
{
  docRes.AddMainDocumentPart();
  var sb = new StringBuilder();
  using (var stream = new StreamReader(docSrc.MainDocumentPart.GetStream()))
    sb.Append(stream.ReadToEnd());
  var date = DateTime.Now;
  sb.Replace("Number", "123");
  sb.Replace("DateS", date.ToShortDateString());
  using (var stream = new StreamWriter(docRes.MainDocumentPart.GetStream(FileMode.Create)))
    stream.Write(sb);
}

At the same time, all content moves normally, but the styles of the document fly off. Maybe I'm doing something wrong?