Replace -tag in the_excerpt

Question

You should try this:

function replace_tag($string){
    $search = array('<p>', '</p>');
    $replace = array('<h2>', '</h2>');
    echo str_replace($search, $replace, $string);
    return $string;
}
add_filter('the_excerpt', 'replace_tag');

or this:

function replace_tag($string){
    $replace = array(
        '<p>' => '<h2>',
        '</p>' => '</h2>'
    );
    $string = str_replace(array_keys($replace), $replace, $string);
    return $string;
}
add_filter('the_excerpt', 'replace_tag');