If you only read the manga, Hange has yet to be given a gender, so...
The same author wanted everyone to believe that Armin could possibly be a girl as well. For whatever reason he likes playing with gender, making Mikasa very masculine in her mannerisms. Then you have the whole is or isn't Reiner gay? Even if he decided to reveal Hange as a guy in the next chapter the anime doesn't have to change Hange into a dude, she could still remain a girl and just have that be a different dynamic than in the manga, it wouldn't be the first time a anime differentiated itself from the source material by altering a character.