I'm not sure if new APs are for sale at present, but as for your second question, yes, you can bridge the gap in price from a regular ticket to an AP, but only if your original ticket has a face value printed on it. Tickets that are sold as part of a package deal do not qualify as they don’t have a cash value. I saw on another thread that you've booked at Hotel New York, so at a guess I'd assume that your passes are part of that package, so I'm afraid you may be out of luck on this.
Thanks for the reply. I thought that may be the case because of how good of a deal I thought it was with tickets included. It was like only £200 extra to add the 4 day tickets for 3 of us.