<div id="olderpost"><a onclick="$.ajax({ dataType: 'script', url: '/actions/olderpost/20'}); return false;" href="/actions/olderpost/20">View older actions</a></div>

$('#olderpost').html().replace('20', '30');

$('#olderpost').html($('#olderpost').html().replace(/20/g, '30'));

<div id="olderpost">
    <a href="/actions/olderpost/20">View older actions</a>
</div>

$('#olderpost a').click(function(e) {
    e.preventDefault();
    $.ajax({dataType: 'script', url: $(this).attr('href')});
});

$('#olderpost a').attr('href', function(i, v) {
    return v.replace('20', '30');
});