code: public ArrayList<String> vps_serviceName_upgrade_GET(String serviceName, String model) throws IOException {
  		String qPath = "/order/vps/{serviceName}/upgrade";
  		StringBuilder sb = path(qPath, serviceName);
  		query(sb, "model", model);
  		String resp = exec(qPath, "GET", sb.toString(), null);
  		return convertTo(resp, t1);
  	}
docs: Get allowed durations for 'upgrade' option
  
  REST: GET /order/vps/{serviceName}/upgrade
  @param model [required] Model
  @param serviceName [required] The internal name of your VPS offer