Código HTML:
<input type="text" id="s" name="q" value="" />
<input name="search_in" type="radio" onChange="changeto('q');">Google
<input name="search_in" type="radio" onChange="changeto('s');">SoftwareP2P
Código PHP:
function changeto(type)
{
var inp=document.getElementById('s');
inp.name=type;
}