Change is coming and we cannot stop it. Think of what the heart of the princess franchise is about: consumer goods. It always has been (look it up).
I do think it is a bit unusual for Disney to mess with their recipe, but as someone else has mentioned, they don't limit the evolution of their designs for successful characters. In fact, when a character gets a tweak or redesign, that's usually a sign of longevity.
I found a link that shows off the new princess dolls:
http://d-princesses.livejournal.com/1335983.html
Check out that header image for the 2012 style.
Some full body shots from this t-shirt.
As you can see, the changes are subtle. Mostly with the hair, slight changes in the costumes. This has everything to do with merchandising. How many Cinderella dolls can you sell when they've looked the same for more than 60 years?