Man I did not mean for this to turn into an essay. Wrote this as a response for the GR m/m group in their surprisingly contentious Historical Gay Romance thread:
I think you and me are talking about three or four different things. To clarify I'll explain how I see magic as it relates to historical settings.
First of all, before I start, anachronisms. This is a problem with all historical novels, not just ones with magic. An anachronism in a magical historical setting would be something like a Victorian werewolf referring to his pack leader as his "Alpha". The term "Alpha" (as it relates to wolves) was invented just a few decades ago to refer to dominant animals (it's also incredibly inaccurate terminology based on some terrible experiments, but that's another rant for another time). In Victorian times it would have simply meant the first letter of the Greek alphabet, and any Victorian werewolf would have just confused the person he was speaking to. "Alpha" is a modern anachronism. Other anachronisms would be a proper society lady who refuses to wear corsets, or crossing the Atlantic in a matter of days prior to the age of the steam engine. This right here is (generally) what I am talking about when I'm saying "I want the book to be historically accurate." I am talking about a book not having modern anachronisms. If there is a difference it has to be explained (like magic sails that make the ship go fast) otherwise I'm going to assume that it is a mistake on the author's part.
Now I've gotten that out of the way. In my opinion magic DOES NOT have to change history! Whether or not the author decides that magic changes history is completely up to them. There are:
1. Novels in which magic does not change history. Magic is part of the world, but it is often 'hidden' or perhaps weak to modern technology, or limited in some way. Or it is used in such a way that the results are the same. Generally when I read these I'm expecting that history is going to flow in more or less the same way, allowing me to imagine the current modern world just as it is (but with magic). These can be historicals (in my opinion, perhaps not for others) because they deal with a history that could potentially be ours.
2. Alternate histories. This is actually an entire genre of its own. Sometimes it's magic that changed history, sometimes it's a new technology. Harry Turtledove is a master of this genre, and I say this a person who does not particularly like his books (generally waaay too many view points for me to keep track of!) These can be really good, but the author has to understand the era he is writing about in order to write a good alternate history in my opinion. What's the point of fighting the US's Civil War on unicorns if the author doesn't understand what happened at Gettysburg? Alternate histories can be a really good way to examine history from another angle and to think 'what if'. What if the South got AK-47s from some time-traveling racists? What if aliens invaded during WWI? etc. But they aren't historicals, because they deal with a history that isn't ours.
3. Fantasies with a historical dressing. I think this is what you are talking about. I honestly have nothing against them, I've read quite a few that I've enjoyed. Most fantasies are of the Medieval European variety, which is a historical setting to be sure. However, these are not true historicals. They are fantasies that take place in a world that is not our own. Again, nothing wrong with that.
A historical, to me, means a novel that takes place in our history. When there is magic the author's intent is important to consider when deciding whether or not the book is supposed to be a historical. Is it clear that the author meant for the novel to be part of our history? Then the book is a magical historical and should adhere to historical accuracy. If the author intends to write an alternate history or a fantasy then fine, I have enjoyed all of those, but they should not (in my opinion) be called historicals.