Providing I class is still available they will be able to reprice the ticket accordingly. If you're happy to proceed with the addition fee they can then reissue the ticket accordingly. Just make sure you tell them exactly what you need (ie please can you reprice my current ticket from P class to I class and tell me how much that will be). Keep in mind it will be any fare difference between P and I plus any change fee according to the fare rules of your original P class ticket. Any fare band in Business except P earns BAEC Avios and tier points.