Please excuse me if this solution does not make any sense
as I do not do a lot of web development and my Javascript
is a bit rusty. But it would seem that you need to have your
onChange() javascript function do a something like this ...

function myOnChange(s) {
var o = s.options[s.selectedIndex];
var t = o.text;

document.location = "whatever?setButtonTo="+t;
}
[download]

where whatever?setButtonTo is a cgi that will re-write
your form with the appropirate value for its submit button.

Hope that helps :)