That functionality isn't in Q2A at the moment but it's a good idea, maybe gidgreen with implement it.
I guess you could edit the comment manually in the database* but it would be easier for now to just hide the comment and post the answer.
* For that you'll need to find the row and change the parent to be the question ID (that's the number in the URL i.e. 2360 for this one) and also change the type from 'C' to 'A'.