Without editing anything hardcoded in the php files there is just one simple SQL query to do it all, can be executed in phpMyAdmin:
SET meta_value = 'default'
WHERE meta_key = '_wp_page_template';
Hope it may help some people!
The file needs a template header and will only behave as a user-selectable custom page template.
True special templates like page-$slug.php that match the post's slug will only work when they live in the root theme directory.