My question was "how to convert already published post into a product?" but while writing this I found an answer: set_post_type($post_id, 'product');
Now here's a new problem that I can't find an answer to: when a post is converted into a product, the product looses all of it's categories and tags.
How can I keep post's original categories and tags? (or) Is there a way to convert post's categories & tags into product's?