That's because at the time writing the idea that a woman could be a warrior was radical and out there - when Tolkien was born women where considered the property of their male relative, and he was 26 before (some) women got the right to vote in the UK. Hence, Dernhelm is Eowyn and can kill the Witch King because she is no man is treated as a big surprise in the book, and was intended as a radical pro-feminist statement. For a modern audience this is obvious, and in the movie Peter Jackson doesn't try to conceal "Dernhelm's" identity.
Now, they could have gone with the "full caster" approach, but Galadriel killing the ice troll with Guiding Bolts would have made for a very different, much more high magic tone. Tolkien's view was magic is subtle. And subtle doesn't work well on TV and film. Which is why the scene in the Extended Edition were Saruman casts fireball is so bad.
So yes, they made changes in Galadriel, but they were necessary, given the different time and the different medium.