There isn’t currently a way in the UI (though plenty of requests for it if you want to add your voice/vote 
Add the option to change a topic to Post Voting format by admin/staff)
I believe it is possible using the rails console. I think you need to change the topics.subtype to ‘question_answer’, though I haven’t given it a runthrough recently to double check if that’s all that’s needed, so you may need to test first.