Just try "php_flag register_globals on" in .htaccess. If it works, then it works, if it doesn't, start rewriting your code.
Again, it's better to just use autoglobals like $GET['var'], since it's guaranteed to work regardless of server config.
Plus, you don't have to rewrite every instance of a variable, just do something like this at the very beginning of the script:
if(isset($_GET['postcode']))
{
$postcode = $_GET['postcode'];
}
else
{
$postcode = "";
}