I have a couple of plugins that allow me to a: http://www.nkuttler.de/nkmovecomments/ move single comments including changing the threading one page at a time (useful for when a commenter doesn't put their comment in the right thread) and b: http://www.dountsis.com/projects/move-comments/ move multiple comments at once to a different post.
I have a very popular poll page http://www.general-election-2010.co.uk/who-will-you-vote-for-in-the-2010-general-election-poll.html that is getting way over half the comments for the site, since yesterday I've been using the two plugins to spread the comments around, (last night the poll page had over 1,200 comments) basically whatever the topic of the first post in a comment thread is determines where I put it, but it's a PAIN, it's taken about 10 hours to move about 600 comments around!!
Using the second plugin I have to search for the text within each comment of the entire thread, so if a 20 comment thread I have to manually find all 20 comments (searching for text within), select them and move them to the new post. It's very easy to mess up, people quote comments and the technique is prone to error which messes up all the threading (miss one comment in the thread and the comments thread messes up).
I'm looking for a plugin that will allow selecting of the first comment in a thread and it will move all the comments in that thread to a post I select.
Hope that makes sense.