Toggle between two classes in jQuery

I'm trying to modify the icons for the jQuery UI portlets. Rather than have a plus to minimize and a minus to expand, I wanted to switch them.

I've only had limited success with it where the first time to click the minus it flips to a plus, but the plus never flips to a minus. Any help on this would be much appreciated.

Here's a sample HTML:

<script src="scripts/jquery-1.3.2.js" type="text/javascript" ></script>
<script src="scripts/ui/ui.core.js" type="text/javascript"></script>
<script src="scripts/ui/ui.sortable.js" type="text/javascript"></script>

<div class="column">
    <div class="portlet">
        <div class="portlet-header">Links</div>
        <div class="portlet-content">Lorem ipsum dolor sit amet, consectetuer adipiscing elit</div>
    </div>
</div>

Here's what I came up with for the jQuery:

<script type="text/javascript">
    $(function() {
        $(".column").sortable({
            connectWith: '.column'
        });

        $(".portlet").addClass("ui-widget ui-widget-content ui-helper-clearfix ui-corner-all")
        .find(".portlet-header")
            .addClass("ui-widget-header ui-corner-all")
            .prepend('<span class="ui-icon ui-icon-minusthick"></span>')
            .prepend('<span class="ui-icon ui-icon-closethick"></span>')
            .end()
        .find(".portlet-content");

        $(".portlet-header .ui-icon-minusthick").click(function() {
            $(this).removeClass("ui-icon-minusthick");
            $(this).addClass("ui-icon-plusthick");
            $(this).parents(".portlet:first").find(".portlet-content").toggle();
        });

        $(".portlet-header .ui-icon-plusthick").click(function() {
            $(this).removeClass("ui-icon-plusthick");
            $(thi