Forum Discussion
Copying word content into content controls in new word document
It is complicated. The content controls as you call them are added to documents by the SharePoint https://docs.microsoft.com/en-us/previous-versions/office/developer/sharepoint-2010/aa544101(v%3Doffice.14) on upload to the library. This stamps the content type into the DOCX and makes the content control available in the DOCX, either through quick parts of the custom XML part.
I don't think there is an easy way of adding the content type structure to your old docs (without uploading to SharePoint or building some custom code) - but I'm happy to be wrong on this.
The trick is mapping any old content controls to the actual quick parts that represent the content type fields. This can be done programmatically but you need a dev that understands the OpenXML API.
It can be more complicated again if using MMD fields and the Term Store as there a hidden list involved at the site level and the GUIDs need to match inside the DOCX.
We've solved similar problems in the past and unless you're looking to fix hundreds or thousands of docs, it might not be worth the effort to develop.
I hope this helps.