Convert custom fields to post categories

Does anyone know how to convert custom fields to categories? And also assign the converted categories to the proper post? This would have to loop through all the posts somehow. Does anyone know of a specific plugin that can do that?

It’s a good question. There is many ways to apply a little script but the simplest way to do this take a few lines.

A note about the following code:

Most parameters might be added to the query like meta_query, category.
wp_create_category(), will create a category once.
wp_set_post_terms(), only work with native post types, will append the new category to other existing.

function wpse_248054(){

    $custom_field_name="_credit";

    $posts = new WP_Query(
        array(
            'posts_per_page' => -1,
            'post_type'=> 'post',
        )   
    );

    foreach($posts->posts as $post){

        $meta = get_post_meta($post->ID, '_credit', true);

        if(! empty( $meta )){
            $term_id = wp_create_category($meta, 3); // 2nd argument, parent id (optional)

            $new_post_cat = wp_set_post_terms( $post->ID, $term_id, 'category', true );

        }
    }
}

add_action('admin_init', 'wpse_248054');

But sorry, I don’t know a plugin to do it 😉