In general, I would write a new question: significantly changing an existing one is likely confusing to users that have interacted with it and it feels wrong as soon as it is voted on or it has answers.
Of course, in this case it makes no big difference — there is currently one vote on the question and no votes on the only answer (a community wiki with you as the only contributor). Nevertheless, I feel both the question and the answer can be useful as they currently stand.
I am unable to think of any compelling reason to ever prefer significantly changing an existing question over posting a new one. Questions that turn out to be useless can be closed as off-topic. Edits that significantly change the meaning of posts make their history needlessly harder to read.