Example form:

<rect onclick="change('blue')"/>
<rect onclick="change('red')"/>
<a id="fred" xlink:href="colour.1?colour=red">
 <text>Go!</text>
</a>
<script>
function change(colour) {
 fred=SVGDoc.getElementById('fred')
 fred.setAttributeNS(xlinkNS,"href",
   "colour.1?colour="+colour)
}
</script>