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.
Langdon. Langdon is the third generation male to own the family library. But what if Langdon was a woman and there's a sudden pressure to live up to the men that came before her? And what about the scene where Freya has to carry Langdon out of trouble? If they were both women that's an interesting scenario. But if Freya becomes a man then it's another dull scene of the man, rescuing the woman and taking her away from danger.
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,