AP discounts are sometimes available for Port Orleans, but as with all WDW resorts often only in limited quantities or specific room categories, and when they are gone, they're gone. Give WDW a call now and see what offers they've currently got available for your dates though. Remember that you won't be "applying your AP discount to your existing booking" per se, more like switching to a new booking which offers a better price.
You can buy your APs during the same phone call, and use them for your discounts, FastPass+ bookings, etc. before you activate the vouchers at the park gate. I just bought ours ready for our August trip for example, and got the special 13-months offer which will cover us for the following August too. Not sure if they can refund an existing paid ticket to the purchase though, or if you need to do that at a park Guest Relations.
- The Unofficial Visitor's Guide to the Port Orleans Resorts in Orlando, Florida -