The distinction between comment and answer is merely the value of the field type in the database table qa_posts. To convert all comments to answers do this:
UPDATE qa_posts SET type='A' WHERE type='C';
To convert just the comments for a particular question do this (replace 42 with the actual post ID of the question at hand):
UPDATE qa_posts SET type='A' WHERE parentid=42 AND type='C';
Edit: For converting comments on answers you can use a slightly modified version of the query for converting comments on questions that I posted in the comments. Just change the parent type from "Q" to "A".
UPDATE qa_posts c INNER JOIN qa_posts p ON c.parentid = p.postid SET c.type = 'A' WHERE c.type = 'C' AND p.type = 'A';
If you want to convert comments for a particular post add a clause for the parent post ID:
... AND p.type = 'A' AND p.postid = 42;