Well I started working on what was going to be a southern gothic horror story that has now. Turned into a Southern Gothic black comedy/horror Oddly I didn't start it out with that intention when I started writing it ever had that happen its new to me.
I just let the story take me wherever. Most of the time I end up liking it better than the original vision anyway. Kinda startling at first, but you very rarely have full control.
Well I just didn't see it going a black comedy direction that it has taken yeah it was startling at first.
Genres are marketing tools, nothing more. Write the story and don't worry about how it will be classified.
Happened to me with my latest project. I wanted it to be a romance, but it's gearing towards a tragic tale.
As long as the shift is organic and not forced its fine. ie you don't want a Gothic horror for half the novel then all of a sudden whoosh an alien space ship appears and the next half is action sci fi (extreme example i know )
Just let the story take you wherever it likes, I guess. I once started writing a short story I intended to be an action sci-fi, but finished it as a philosophical sci-fi drama. I kinda got to keep the sci-fi part, though.