Warbler said:

M was originally played by a man, and M was a man in the books.

Yes, I am aware of that, but, again, it's a title, not necessarily a specific character. Therefore, it seems that they are open to changing facets of "M" when it's a different "M." Believe me, I feel the same way you do. When Justice League first came on, and Green Lantern was a black man, I was a bit annoyed that they were being politically correct and changing the character's race. I eventually learned more about Green Lantern (not knowing much at all) and realizing that there have been several Green Lanterns, and one of them was black, so it's not so much of a big deal to me.

Let me see if I can be clearer. I know there's a fan theory that even James Bond is just a title used for several different spies in order to rationalize how Bond can continue on for so many decades (and obviously look so different in various appearances). But let's forget about that. If we are to assume that Bond is a singular character, that there is only one James Bond, then, yes, I would feel it to be quite irritating if he suddenly became a middle eastern woman and treated as if he/she was always that way. But conversely, in GoldenEye, they specifically address the fact that Judi Dench's M is a brand new character with references to her male predecessors. New person, doesn't retcon the old Ms, so I'm okay with it.