Changing the template hierarchy

Try this filter

function wpse_63614_restructure_template_hierarchy( $template ){
    return get_template_directory().'/filename.php';
}
add_filter( 'template_include', 'wpse_63614_restructure_template_hierarchy' );

The filename would be the file you would want to call in your theme folder. For child themes you would use get_stylesheet_directory instead.

EDIT:

Or as suggested by Chip Bennett and what I too feel would be better; you can use the template_redirect hook in the following manner. The priority can be set accordingly if required.

function wpse_63614_restructure_template_hierarchy(){
    include( get_template_directory().'/filename.php' );
    exit;
}
add_action( 'template_redirect', 'wpse_63614_restructure_template_hierarchy' );