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// Change default position of 'featured image' meta-box
add_action('do_meta_boxes', 'relocate_featured_image_meta_box');

function relocate_featured_image_meta_box(){
    remove_meta_box( 'postimagediv', 'YOUR_POST_TYPE', 'side' );
    add_meta_box('postimagediv', __('Featured Image'), 'post_thumbnail_meta_box', 'YOUR_POST_TYPE', 'normal', 'high');
}