I personally would not do it. Also, during the downtime periods, we smods are as dead as any other member, only admins can work then.
But I'll show my reasoning: the result would be a 20000 posts-long beast. I personally instaured the tradition of closing threads in my old boards when they hit 30000 posts, so of course I'm not a fan of keeping such long ones, specially when the new one is light and small. You can just keep a reference to the old thread in the first post.