Refreshing existing documents to better align with updated templates is a complex puzzle. Importing the template styles is really just the tip of the iceberg but it is a relatively safe thing to do. Other things you might choose to update have the potential to lose document-specific information or require a huge amount of adjusting specific items.

Lets say you amend the template to:
1. Change the cover page layout
2. Change the front matter (Table of contents field, document history table, acronym list, approvals table etc)
3. Change the header/footer layouts
4. Change the page setups
5. Change the table formatting

For all of those template-level changes, it would be highly problematic to just automate the updating to match the new template. Unless you have religiously used mapped Content Controls (linked to document metadata) in all those legacy docs then you can't just paste in header/footer/cover/front matter without also needing to re-instate the document specific information. For instance, the title of the document is different for each document so pasting in a template version of the cover removes the document title.

Similarly, changing the page widths to match a template page setup change means you may also have to visit the table and graphic widths for all those items.

It is absolutely POSSIBLE to create macros to adapt existing documents but if you are only just now asking about how to do it then it is too late. Existing document setups have to be configured in such a way that you already thought about needing to update the template BEFORE you actually need to do it.

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Andrew Lockton
Chrysalis Design, Melbourne Australia