1 - fa un backup al fisierului ce contine functiile.
2 - modifica functia cu asta:
function previous_post_link($format='« %link', $link='%title', $in_same_cat = false, $excluded_categories = '') {

if ( is_attachment() )
$post = & get_post($GLOBALS['post']->post_parent);
else
$post = get_previous_post($in_same_cat, $excluded_categories);

if ( !$post )
return;

$title = apply_filters('the_title', $post->post_title, $post);
$string = '<a href="'.get_permalink($post->ID).'" title="AICI_VARIABILA_TITLU">';
$link = str_replace('%title', $title, $link);
$link = $pre . $string . $link . '</a>';

$format = str_replace('%link', $link, $format);

echo $format;
}