Honestly, that has "scam" written all over it. Luring people into purchasing packages or rooms, thinking that they're getting 40% off, yet when the bill comes, they have to pay it in full. Disney needs to start being honest and straightforward with people in every aspect (not just prices), or they're going to start to loose credibility and customers.
Well, WDW can't MAKE me pay anything. I have a confirmed rate with (printed) proof of what they told me. If they try to tell me I have to pay full rack rate, worst case scenerio, I chargeback the monies already paid with my credit card (in full), and I will easily win - and I simply don't go to DW. I go somewhere else instead.
My trip will have been paid in full 45 days before I go. There is no way for them to "charge" me more at check-in, unless I give them a credit card. If they refuse me a room when I have been paid in full for 45 days, I get my lawyer (uncle) involved. And, go to a Holiday Inn and use my dh's points for work (for free lodging) in the meantime.
If an invoice comes before I pay in full, and it is for more money than agreed, I will talk to whoever I need to, and if necessary, will chargeback any monies already paid and change my vacation plans. They clearly can't keep my money if they change the deal. Of course.
WDW will need to decide if the $500 or so is worth losing a loyal customer for life. I will still visit WDW, but not on-site and certainly not spending all of my vacation dollars with them. It is simply a business decision. And, I will be clear as to what is on the line.
If you look at this as business and finances, and not emotionally, it becomes very clear. I love WDW, but as a business owner, I know I can't promise my customer one thing, in writing one day, and then say, "Nope, I lied. You can't have that..." and expect them to just accept that. If I do deem the loss as more important than their brand loyalty - well, that is a choice I make. And, I accept the consequences.