Everyone told me "Go in September, there's no one there" so we booked a trip and will be there next week but I hear all the values and moderates are FILLED TO CAPACITY. I'm not sure this is true but when I call to try to get one more night at POP (and have been calling for 3 months) there's "no room at the inn". Reservations tells me all resorts are filled except for deluxe.
Maybe try a different tactic. Call tomorrow. Don't tell them you want to add a day to your reservation. Tell them you want to make a new 1-night reservation. Here's why:
Let's use dates, to make it easier. Let's pretend you have a room booked for November 1-7, and now you're hoping to add November 8. If you call up with your reservation number for your 7-night stay and you want to add November 8th, they aren't looking to see if there is availability for November 8 ... they are looking to see if there is availability for November 1-8 in your exact room category. The problem is this: maybe, in this pretend example, November 3 is sold out for some weird reason. If November 3 is sold out, then they can't book a reservation for you from Nov. 1-8. It doesn't matter that you already have a room reserved for November 3 ... if they add a day onto that reservation, they have to rebook that reservation in their system (it will use the same reservation #, but they still have to re-do the entire thing).
So instead, try to book a separate 1-day stay for November 8 and see what happens. You'll make this a room-only reservation, you'll have to pay for it up front (because the payment terms for a room-only reservation is 1 night's room rate + tax), and even if it's in the same room category at the same resort you'll still need to go to the front desk to check-in again on the morning of the 8th, and if you're very unlucky you'll also need to vacate your old room and move to a new one.
But please, try this strategy tomorrow and let us know how it goes. Try the 1-day reservation. As long as you are more than a week before your trip, you can wait up to 7 days between the time you make a room-only reservation and the time you pay the deposit ... in other words, if you change your mind, you can cancel it without obligation, or you could just not pay for it and it will auto-cancel.