Convert WordPress posts to products in WooCommerce

You could install the Post Type Switcher plugin

Or Run SQL query on your database to change the post to product post type

UPDATE  `wp_posts` SET  `post_type` =  'product' WHERE  `post_type` = 'post';

Backup your DB 1st.

UPDATE  `wp_posts` SET  `post_type` =  'wpsc-product' WHERE  `post_type` = 'post';