gistfile1.txt

function alter_my_meta( $check, $object_id, $meta_key, $meta_value, $prev_value )
{
    // Avoid infinite running of filter
    remove_filter( current_filter(), __FUNCTION__ ); 

    // Do whatever to alter the meta value 
    $meta_value = strtolower( $meta_value ); 

    // Update with new value
    update_post_meta( $object_id, $meta_key, $meta_value, $prev_value ); 

    // Return something else than null
    return true; 
}