Trying to update an OnMouseOver...(JS)

[Static]

here is my JS function

<script type="text/javascript">
	function prevHover() 
	{

	  var a = "this.T_WIDTH=400; this.T_BGCOLOR='#000000'; this.T_BORDERCOLOR='#666666'; this.T_FONTCOLOR='#FFFFFF'; return escape('" + document.getElementById('longd').value + "');";  
    document.getElementById('preview').onMouseOver = function(a); 
	}
</script>

here is the HTML line its supposed to hit

<a id="preview" style="cursor: pointer; cursor: hand; color: #FFBF00;" onmouseover="this.T_WIDTH=400;this.T_BGCOLOR='#000000';this.T_BORDERCOLOR='#666666';this.T_FONTCOLOR='#FFFFFF';return escape('');" target="_blank">HOVER FOR PREVIEW</a>

basically i just want to update the onmouseover...
specifically the part in between the single quotes here return escape(''); but I know I cant just hit that...

now If I just do an alert() of what the current mouseover is.. it works.. but I cant seem to set it!???

thanks

BTW: that code is calling another JS Script that creates a popup