What's the best way to optimise 8year old pages that don't have metadata or specific keywords?

I am migrating an 8 yo website from Weebly to WordPress and most of the pages don't have metadata nor specify their keywords. The website's SEO is not too bad considering this, but I am afraid that, when adding a proper metadata and assigning keywords, it will be bad for SEO. Do you have any recommendations on how to optimise the pages?

Thanks in advance!