Freya. Without going into spoilers, Freya is a physically and emotionally strong person. In fact she's the strongest of the group leading her to be the first to fight and feeling as if it's up to her to defend others.
If you change her to a man, then Freya is closer to the tired trope of the male hero. You try and think of the "strong male hero" and you'll come up with thousands of references. You try the opposite and that number goes down drastically.
While I haven't touched on it too much in book 1, there is/ will be a frustration from some male characters that a woman is far stronger than they are.
Liz. The role of the IT tech whiz is usually given to a male character. So again, by making Liz a male character you turn her into a tired trope. Liz is also far more feminine than Freya, she's caring and though she wants to protect people, she is quite scared of the idea of fighting. So what if Liz was a male character and kept those traits. That becomes interesting again. Would he be judged more for being afraid? Would he have insecurities about having such a strong woman around or is he quite comfortable in himself?
Matt. I hadn't really thought about it before but werewolves are another creature that default to male quite a lot. I actually can't think of a famous female werewolf (edit: I just remembered Once Upon a Time's Red, although the whole point of her is to switch around the story of Red Riding Hood). Matt's big moment is where he has to regain control of his wolf side after something traumatic happens. Would people judge a girl differently for losing control? Or would anyone feel even more protective of her?
Matt's father and brother are also important to his story. If Matt was a girl growing up with overly dominant male figures, would those relationships change? Probably. There might be a bigger resistance to him leaving home. The mother might even be more inclined to get involved.
Gender is just one aspect of a character but it's a big one. Changing it might change how other characters treat them, or it might change how they react to certain situations. It also changes how the audience perceives them. A girl trying on makeup isn't a big deal but a male character does it and suddenly you have a situation that goes against the "norm".
When I started writing Darwin Solution I was a very different writer to who I am now. This little exercise has shown that maybe there were some interesting avenues to explore. I question things now and challenge myself to ask more of the "what if" questions. Because when you do, interesting things can happen.