<select id='derp' onchange="changeVal(this,'changeme');">
<option value='test'>test</option>
</select>
<input id='changeme' />
<script type='text/javascript'>
function changeVal(el,changeElID){
var changeEl = document.getElementById(changeElID);
changeEl.value = el.options[el.selectedIndex].value;
}
</script>
[EDIT] re-reading the question, it looks like you are trying to change the name of the input field ... if so, change changeEl.valuetochangeEl.name
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