I just got off the phone with Cloe in Passholder services. You can pay the difference from your existing rate of purchase (she said $563, I am not sure that is right) to $649 plus tax and I would get photopass for the next trip. There is no prorated amount (Since my original post is probably five pages back by now, I asked because our AP expires Dec 13 and based on today, we won't be renewing). Also, you can upgrade only one pass however, to get the photos, THAT upgraded pass needs to be scanned. Meaning, since we have four APs if we only upgraded one (roughly $90), we would need to scan that pass at all Photpass Photographers.
She said I would have to upgrade in person.