I believe so - and I think that the existing course content will be completely replaced by the content of the template course. The page where you upload the CSV includes several options for creating and/or updating courses:
And I'm in total agreement with Rick on having access to an experimental site. I use MAMP on my Mac, too. And Moodle.org has links to installers for both MAc adn Windows on the latest releases page - https://download.moodle.org/releases/latest/. Plus, my organization (a K-12 school district) provides me with a "live" test site. It is a copy of our production server. I use it to try things out (like uploading courses) or test new plugins. And the real techs use it when it is time to upgrade, as a place to hold a copy of the production site, I believe.
David