Good question! I was wondering about the opposite. Is it possible to turn on votes to a post that has just been created and has no responses yet?
I arrived to this question after what could be considered a minor UX bug: if you toggle the topic to a voting post and then change the category, the toggle goes silently back to a normal post, and this is easy to miss if you aren’t used to create voting posts. Since the toggle button is on the top left of the editing window, it is likely that users will toggle before setting the category, following the usual top-down flow. This would not be a big deal if the normal topic just created could be changed to a voting post, but I could not find a way. I had to delete the post and create a new one, this time setting first the category and then the toggle.