Should I delete an old question with a wrong answer?

Shifting my moderator hat backwards here, and speaking more from a personal perspective than a moderator capacity.

I think that our culture where errors are swept under the rug is problematic, and a lot can be learned from mistakes. Especially good mistakes, where the issue is subtle, interesting, and can teach us something.

I'd be happy to see you edit the question. You don't need to even unaccept the answer. We can freeze it in time, if there's any issue, as a historically important post.

Then, post a new question. Give context, backlinks, and an explanation, and then in the future, someone who stumbles upon the old question will be able to learn from it something just as valuable as the "neat and clean answer".

(In short, we don't teach students enough about failure and reattempts, not publicly anyway, and I think that's wrong. Not advocating sending students and colleagues to fail, but almost everyone will at some point, and this is a great opportunity to show how it can be done with grace and aplomb.)