According to
this article, it appears it was for her “coronation” as the 11th Disney Princess ™.
Here’s the statement coming from Disney:
“The artwork used on Merida’s official social media sites has always been the imagery from the movie. We routinely use different art styles with our characters and this rendition of Merida in her party dress was a special one-time effort to commemorate her coronation. Merida exemplifies what it means to be a Disney Princess through being brave, passionate and confident and she remains the same strong and determined Merida from the movie whose inner qualities have inspired moms and daughters around the world.”
Many of the articles are saying Disney “pulled” the new image from their website and put the old one back up, but Disney is saying they never changed the image to begin with.
I’m inclined to believe the Disney rep. My daughter has princess stuff all over the place. It’s not uncommon for
any of the Disney princesses to appear in different art forms here or there. It’s just what they do. It doesn’t mean they are going to go back and expunge all the old images, though. The original image will always be the standard.