In the edit dropdown, there's now a "Move message" link (when it's available) that will let you move a message to be a reply to the original post. I may enhance this in the future to let you move a post anywhere in the thread, but the UI for that would be much more complicated.

http://cougarboard.uservoice.com/forums/202670-general/suggestions/5196723-under-edit-you-have-a-button-to-move-to-origin