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. Change the post type from "Q" to "A" and also change the parent ID of the comment to the parent ID of the answer (i.e. the ID of its grandparent, which is the question).
UPDATE qa_posts c INNER JOIN qa_posts p ON c.parentid = p.postid SET c.type = 'A', c.parentid = p.parentid 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;