Template updating for old documents

Good day! I ask you to give me support in resolving my issue.

I have a document library in SharePoint site. I created content type inherited from document content type. Created content type has additional fields. I set document template for created content type. This document template is Word with quick parts from additional fields. Anything work fine. Later, I had to add some fields to a content type and had to update document template. New documents are created on the new template. But in old documents, that were filled in past is used old template. How i can update template for old documents? Thanks!