No. The thread alone doesn't have to be active. It's just that your post itself must be before the first post of the thread time-wise that you want to take over.
So if that post, that you linked, is made after the first post of the Chorus Kids thread(which seems to be the case by the wording you gave), I can't move it that way. It's not just a post that you don't care about that. It's a matter of time/dates. A post that exists before the Chorus Kids' first post, basically.
...I wish I could explain it better. >.<