What is the best practice workflow for new website overhaul and keeping current SEO ranking? [closed]

If you are only changing the theme or plugins, then it shouldn’t make a difference at all. I see no reason to create new posts and/or pages just because you are changing themes.

If you are doing a complete site overhaul, however (you don’t specify), then yes, you should add 301 redirects to your .htaccess page.

The basic structure of a 301 redirect is:

Redirect 301 old_address new_address

Each redirect should go on its own line.

A 301 redirect means that the page has permanently moved (as opposed to a 302 redirect, which reflect a temporary move). This is generally considered to be a good procedure by search engines, and thus is the recommended way to handle changed URLs. Additionally, anyone who has bookmarked some pages in the past will automatically be redirected to the new pages without getting a 404 error.

FWIW, Google does recommend the use of 301 redirects.