http://api.weixin.qq.com/cgi-bin/changeopenid?access_token=access_token
返回空字符串，http状态是301重定向，我用的是php的curl工具调的

function http_post_json($url, $jsonStr)
{
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $jsonStr);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array(
            'Content-Type: application/json; charset=utf-8',
            'Content-Length: ' . strlen($jsonStr)
        )
    );
    $response = curl_exec($ch);
    $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
    curl_close($ch);
    return array($httpCode, $response);
}