See, I don't get that viewpoint. For it to happen, two characters would have to pull a total reversal and Jim would have to undo a whole lot of character development to contrive it. I'd see that as terrible writing and betraying who the characters are. It would be like Harry suddenly deciding that Murphy is better kept in the dark, despite him having learned that lesson ages ago and repeating as recently as Zoo Day that he remembers it.
How is that something you wouldn't mind?
A couple of things: first, it depends on the circumstances. If it was handled with nonsense plot contrivances that don't make a lick of sense, I'd be furious at the betrayal. If it was motivated by, say, Murphy feeling left out because she can't keep up with Harry, I'd drop being a Murphy fan (which I am) immediately. But if the circumstances were such that they mirrored Harry's in Changes sufficiently, where Murphy was manipulated, beaten down, and needed to grab onto
something to save Harry/Maggie/Someone Too Important to Lose, I could get behind it.
Second, Murphy's attitude about Harry being the Winter Knight (at least as recently as Skin Game) has been "You'll find a way to get out of it" (pretty much my attitude as well), so Murphy might not consider it as permanent as someone else. Harry does try to disabuse her of the idea that it's possible to wiggle free of the Mantle, but I don't know that she buys it.
Third (very weak point coming up), the circumstances might not be such that she has much of a choice. I don't think the Mantle will head to her by default; Summer Knight pretty clearly demonstrates that the Knight's Mantle will snap back to whichever Court it belongs to. Aurora had manipulated matters and circumstances such that the Mantle didn't go anywhere, by hiding it in Lily, who she turned to stone rather than killed. It didn't have anywhere to go since its owner was still alive, just, you know, doing a Weeping Angel impression. But I could see circumstances of someone becoming a Knight against their will, and it might be a good way for someone to screw with Harry/Murphy (such as Sarissa, though I doubt she's holding a grudge).
All that aside, I'm pretty sure the act of will is an important part of becoming a Knight, as it's kinda their whole purpose to begin with (otherwise Mab might've just crammed the Mantle down Harry's throat several books ago). I'm also positive that the Fae aren't capable of violating a mortal's free will in the first place without a deal being made, so again, the above point is pretty moot. I only bring it up to counter other arguments in the thread about it being something that
happens to Murph rather than something she does.
In the fourth place, I wouldn't mind it because I think it might be a fun and interesting dynamic. Fix isn't all that bright, so when he says that the Summer Knight's job is to stop the Winter Knight, I don't really buy it. I mostly think that the job is to prevent the Winter Knight from screwing with mortals, or Summer. It seemed to be the MO of the previous Summer Knight, the artist guy (I forget his name and am currently too tired to look it up) to take care of, essentially, Lost Children. That Changeling gang hung around him because he was (I presume) a pretty good guy who offered them protection from the Fae Courts. I could see Murphy turning the power of the Mantle to becoming a sort of supernatural cop, for example.
Fifth, I like it better than anything else that's been tossed around. Becoming a Valkyrie is... meh. Too morbid for Murphy; they Choose the Slain. I'd prefer she not be a Knight of the Cross, as it was A) way too obvious, B) it didn't turn out so great the first time, and C) it's been done to death already. Vampire love-interest was already done (and is just passe at this point). At least as the Summer Knight she won't become a jerk, either; Fix "Grew" rather than turned into a meathead, and his motivation remained essentially "protect Lily," as it was in Summer Knight.
All of this aside, it is not my preference for Murphy to get any sort of power-up. I want her to remain a vanilla human; she's badass enough as it is. Even post-Skin Game-"CAN'T-KEEP-UP-WITH-HARRY-ANYMORE-OH-MY-GOD-ABANDON-SHIP"-Murphy is ten times as badass as the majority of people in the books. I'd put 90% Murph up against a wider range of supernatural threats in the Dresdenverse than most would, I think. Keep her as is, I say. Let her run the Paranet as she has been.