[defabricator]

CREATE TEMPORARY TABLE firstPosts SELECT
 thread as threadId, min(id) as postId
FROM
 blabla
GROUP BY
 thread

CREATE TABLE neuesBlabla SELECT
 a.id as id ,a.parent as parent,b.postId as thread
FROM
 blabla as a
LEFT JOIN
 firstPosts as b
ON 
 a.thread=b.threadId

blabla ist Deine bisherige Tabelle und neuesBlabla ist dann die fertige Tabelle, die Du nach ausreichender Kontrolle umbenennen kannst.