prova così...
<form name="form1" method="post" action="">
<label>
<select name="select" onChange="document.form1.action=document.location. href;document.form1.submit()">
<option value="a">aaa</option>
<option value="b">bbb</option>
<option value="c">ccc</option>
</select>
</label>
</form>