Really miss this AP when we used it a couple of years ago! DVC discounts were the only way to go with these ....... oh well, maybe they will offer again someday!
Yes you can. You just need to pay the difference in price. Please note though that if you currently own a different level of AP (at DL or WDW) they don't pro-rate the price so you'll pay a lot and the expiration date will not be extended. It may make more sense to just buy an AP at the "other" park.
They really jacked up the price this year to go along with the increase in APs. We had one last year and it was more like $1100, so basically if you had the top AP pass and took one WDW trip it was worth it.