The Master already regenerated into a woman, and the idea that regenerating time lords can change gender has been mentioned a few times on the show in recent years. So they definitely CAN do it. The question is whether they will.
Personally, I don't think it's necessary. If they have an actress who they think is perfect for the role then they should definitely go for it, but I don't think they should force it. In a way, given they only just made the Master female, they might actually be better off waiting for one more regeneration.
I'd actually be more interested in them casting a non-Caucasian actor this time around. There's definitely no reason at all to not do that.