When I've had preorder status on Amazon stuff, I have always gotten email notifications when the price changed. That happened when I ordered the last Harry Potter book on preorder. About two weeks before the book was due to release, they sent me an email saying that they would not be charging as much for the book as originally listed. Instead, the price went down about 10 percent. Of course, it never happened the other way (ie. the price going up). That's simply never happenend.
I think that while Amazon does not always have the best prices, they do have good service and if there is a problem with their products, they are always accomodating with their customers. But, that's just my limited experience with them, and I've been a customer there for the last two-three years.
So, while the preorder status of Fraggle Rock may change, I am sort of thinking that they will not contact their customers and say they are going to charge more for something that was preordered at a set price. I think they have more integrity than to do that...they could also lose customers if they did.