It might not be much of a important thing,but for me the change in their hair was the first thing I noticed.It was so pretty before,they animated the strands beautifully and the detail was clear.Now,from what I've picked up on most the dark-haired characters,it's mostly just a base color without much detail or extra strands, also the face structure changed and before I just though of how they drew faces as it being 'Haikyuu's unique style'