It makes sense to me that they would have Hulk and Tony fight, after how much they were presented as bros in the last one. Also, it seems Scarlet Witch and Quicksilver are going to start out as antagonists.
I bet the change was a big design choice. Not only does it mean not having to deal with wigs or dying, but it officially severs the biggest aesthetic cross over with his comic father, which they have to do considering it's right now impossible for both characters to exist in the same movie universe. Though it does make his name a little silly.I'd prefer a silver-haired Quicksilver, but it's still not bad look.