Form Submission On Option Selection

I want to use a drop-down menu selection option to submit a form. But the form's contents change depending on the option. This is the basic form:
<FORM ID="go1" METHOD="POST" TARGET="_self">
<INPUT TYPE=HIDDEN NAME=UserID VALUE="<%UserID%>">
<INPUT TYPE=HIDDEN NAME=Password VALUE="<%Password%>">
<INPUT TYPE=HIDDEN NAME=SubUnitID VALUE="4266">
<INPUT TYPE=HIDDEN NAME=PageID VALUE="1">
<INPUT TYPE="IMAGE" class="hand" border="0" src="/images/my.gif" width="45" height="26" vspace="5">
</FORM>

At the moment, it's an image which gets clicked on but it needs to be the option chosen. I think I can do that with:
function gotoPage(s){
var v = s.selectedIndex;
var select = document.forms.form.elements.input; // selects question
if (v == 1){...}
if (v == 2){...} etc.

However, as I said, the form's contents must change depending on the option chosen: The form's values which change are:
<INPUT TYPE=HIDDEN NAME=SubUnitID VALUE="4266">
<INPUT TYPE=HIDDEN NAME=PageID VALUE="1">