I've had to do some changes since I started too, as my tastes changed. What has worked well for me is to search for all books with e.g., "fantasy" in their tags, then do a bulk metadata edit.
From there, you can add the "Fantasy" tag you want to all of them, and remove all the extras, like "fantasy epic", "fantasy & fiction", "fiction & fantasy", etc...
It's pretty easy, since just typing the first letters - "fan" - makes all those others present pop up, and you can just select them to delete them, and then OK, and it's done in a swoop.
