I'm not convinced, actually. She just has a ponytail. You can see the tail of it in the close-up. I froze the screen there, and it just looks like there's something strange and light-filled going on... maybe connected to where she disappeared to at the end of Silence in the Library? Hmm. Anyway, I didn't think she looked significantly different, at least not enough to merit calling her alt!Donna. I guess we'll see, though.