yes and no and a work around
You can edit the title of the root post that starts a thread if you are the author (or a moderator).
You cannot change the title of a sub post.
However, the paragraph tag in the body text can allow you to change the subject line later. Make a comment post but don't fill in the subject line. It will default to the first part of the comment to add as subject line. After posting, edit the post by putting paragraph tags around the text you want as the subject line. This needs to be the first line of the post.