There's someone analyse that SW is the one change most after 6 years skipped. Not only his hairstyle but also how he looks. More mature and more neglect.
Before except the time he was late for school and didnt want BR to wait in the cold, SW always walked slowly and looked around. He was the last one came to JH's house and also the last one came out from JH's room in ep2. Whenever he crossed by the Sung-Kim's house, he always stared into their yard to check if BR was there. He never rushed to walk home after studying in the midnight.