Hi IG,
Every document will have to be updated - but presumably you'd only be doing this for new documents anyway, since a change to old documents means they wouldn't reflect the content when they were printed/sent (which could have legal implications), plus changing the logo means changing those document's 'last modified' date. Still, if that, what you want to do, see:
https://www.msofficeforums.com/word/...rate-docs.html
If you're using a template, changing the logo in the template means all new documents based on that template will have the new logo.