If you downgrade to the Free plan, you continue to sell your books and courses on Leanpub.
This will not affect the way your books and courses are presented on Leanpub. They will still be available for sale in the bookstore etc., just like when you were on a paid plan.
Currently, after you downgrade to the Free plan, you will still be able to preview and publish your books and courses in their current writing mode, but subject to the Free plan's limitations on the number of previews and publishes you can do in a month.
After you downgrade to the Free plan, you will also keep your Custom theme settings.