There's a craft to every kind of writing.
Consider different kinds of vocal music. Tuvan throat singing, beatboxing, and singing opera may require different skillsets, but they're all difficult and require countless hours of practice to master.
Luckily for you, there's a lot of overlap. All three types of music above require breath control, an ear trained for pitch, the ability to keep rhythm, etc. If you're a solid fiction writer already, you won't have to learn from scratch, you'll just have to determine how creative nonfiction differs from fiction and develop those particular skills on top of your existing foundation.