You can change their appearance easily, even in game. Pronouns, gender-specific text, etc. would require some modding work, but can be done as well. However, voice lines would absolutely be fixed and unchangeable. Otherwise, we wouldn't need voice actors anymore. :P Though I suppose it's possible to change the pitch of all of Ifan's voice lines using some audio software. I'm not willing to go through that trouble, though. Are you?
Interestingly enough, I'm pretty sure most companions don't refer to their own genders. (ie. Ifan doesn't say, "I'm a man in a beard on a manly mission") so if you really wanted it badly enough, this could probably be done rather gracefully. You just need to put in time and effort.
