Its because of the curse mark on his shoulder that he has that new look. After their first fight on the roof Sauske realizes that Naruto is stronger than him (they both hit a water tower with their attacks thans to Kakashi for stopping them, Naruto used Rasengan and Sauske used Chidori). At first it seemed Sauske did more damage by peircing the water tower and Naruto only dented it, but Sauske later on sees that the entire back of it was blown out by Naruto.
Sauske starts to realize the strength Naruto has and realizes his own weakness. This is when he encounters the Sound Four who were sent by Orochimaru to capture him.
After a short battle and some talking, Sauske decides to go with them. They put him in a special basket that (with proper seals put on it) would release Sauske's curse mark and bring him to a new level of power.
After Sauske leaves, Naruto, Neji, Chouji, Shikamaru, and Kiba head out to find Sauske before he reaches the border. Each of the Sound Four were taken out by one of the team members in individual fights. Each fight was amazing and I suggest looking into them as well. After awhile the fifth Sound ninja appears and fights Rock Lee who appears later on as well.
After all the other fights come to an end, Sauske emerges from the basket with the power of the curse mark unleashed. This is where his appearance changed but with out the wings. The wings come later on in his fight with Naruto.
After Naruto tracks Sauske down, they have alot of talking and some fighting. They eventually get into the real fight and it turns out to be a great one. I think its the best in the series. The second fight turns out into a tie I think. I suggest looking into it alot. This was the best part of the entire of series so far.