Make sure when you informed them, you sent the letter through something you can track (priority mail, fedex etc.). Then it's important to keep following up, whether through infopass or calling the CIS hotline number. The answer to "how long they update" the category is that the CIS in very inconsistent in these requests. Unlike a filing (I-130, 485 etc.) If these requests are not followed up on, they sometimes do not even update it, because you won't get some sort of receipt notice when you send these requests.
That said if the category was switched and the priority date is current, the petitioner would be notified immediately and the interview could range anywhere between 3 to 9 months to even a year depending on the country and consular office's schedule.