how to FeedData to Part or string to MainDocumentPart with .net 4.8

[evlo]

How to FeedData to Part or string to MainDocumentPart with .net 4.8?
This examle does not work https://learn.microsoft.com/en-us/office/open-xml/how-to-search-and-replace-text-in-a-document-part
as i get message Memory stream is not expandable

So far I did not found a way to update/set data of the document part from string in newer .net frameworks.

[evlo]

turns out i was accidentally still initializing wordprocessingdocument by stream with fixed length and any change to any part of the document would result in exception.