If you book online, you should see a link to "change reservation" when you look up your reservation on
MDE. I had booked a rack rate room for the Marathon week, and was able to change it to a passholder rate online. The reservations I've booked over the phone don't usually show this link, so I have to call in for those. But it can still be done. I agree that you have to be flexible and willing to change if the resort and/or room type you have booked now isn't available with a discount. Sometimes for me the resort choice or the dates are more important (I even paid rack once

), and sometimes I can be very flexible if necessary so I can chase the best deal.
POFQ has been available recently for discounts, so I would definitely check on that resort if it's the one that interests you. I have an AP discount there next month and a bounceback discount there in May.