<div class='aa' id='aa'>123</div>
<div class='bb' id='bb'>456</div>
<style type='text/css'>
.aa{
display: block;
}
.bb{
display: none;
}
</style>
<select onChange="check(this)"><option value=aa>aa</option><option value=bb>bb</option></select>
<script type="text/javascript">
function check(s){
	if(s[0].selected){
		document.getElementById('aa').className = 'bb';
		document.getElementById('bb').className = 'aa';
	}else{
		document.getElementById('aa').className = 'aa';
		document.getElementById('bb').className = 'bb';
	}
}
</script>