I manage multiple Word document templates for my company

When I say templates, I don't mean Word templates, but Word documents that contain code that our system, essentially Mail Merges from system data into finished Word documents. So, my resultant document is referred to as a template and my starting document Ive called a Master document

There are many hundreds of these

To make life easier I wondered if there was a way to automatically copy the complex header of a Master document into 10s of Word Document templates

I've attached a current 'Master' document (Master UK.docx) and one example of the Template I need to produce (585prop01.docx).

It's not hard to Ctrl+C and Ctrl+v across from one document to another but often I have 10s (sometimes 100s) of documents to update if the business wants to change something, ie the logo or the layout. (And this is without talking about the Footers or the Page 2 headers which is different

Im looking to see if theres a way to apply some kind of automation to this as it's extremely labour intensive and time consuming. To make it worse the templates are password protected, although these is a 'connection' between the document name and the password