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Setting the TV threads aside for now, let's talk about the movie threads.
The "proper" way to do what you're doing is to merge the "Unlinked" thread into the "Linked" thread. That way the original linked thread would retain it's information and no changes would be needed.
Merging threads the way I mention (unlinked into linked) assumes the "Unlinked" thread was started after the "Linked" thread so posts will still flow in a semi-reasonable manner.
But, there is also several problems with doing what you're doing...
The original first post in the thread contains a "decoded" version of the movie information that gets displayed in the event the add-on is disabled or uninstalled. So, that post no matter where it is in the thread after a merge will still show the poster, etc. in that post.
If you're going to merge a "Linked" thread into an "Unlinked" thread, you should move all but the first post in the "Linked" thread to the "Unlinked" thread. Then delete the "Linked" thread with that single post. That will delete the information from the database. Then add the movie information into the "Unlinked" thread using the "Add information" function.
All in all, if you REALLY want to remove the movie information from the database, it's in the xf_snog_movies_thread table. Delete the row for the "Linked" thread's thread_id in that table. That should do it, but I won't support that if something goes horribly wrong.
TV Threads are a much more complicated system than the Movie threads. I do not suggest or support merging threads the way you are at all.
Overall, I don't suggest merging threads like you are at all. You've experienced just one of the problems that can crop up. The only way I would merge threads is as new threads are created, you can merge them into a pre-existing "Linked" movie thread. Or, if there is no "Linked" thread, ASAP move the new thread into the movie forum and link it so all future posts/threads can be immediately merged into the "Linked" thread.
If the "Unlinked" thread is "older" than the "Linked" thread then I strongly suggest leaving it alone, closing the thread and referring people to the newer "Linked" version for future posts.
I hope that's clearer than mud.
Basically it boils down to "Don't let movies be posted anywhere except in a Movie forum"