You may not have enough time to get a new PP here if you are leaving next month. If you are able to do it here, you'll receive a letter from embassy or a stamp in the old PP regarding the change of PP. Bring your new and old PP to immigration with copies of pages in both, fill out a 'transfer of stamps' form. Immigration will transfer your extension to the new PP if you are leaving the country, if not leaving they prefer to do it at the renewal extension time, I was quoted this by an IO at CW last year.