Actually...just moving the thread...without leaving a redirect, would be the simplest solution...IMHO.
If someone is looking for a thread they started...and it was moved...it should still be listed within their Profile Summary Stats. The actual thread is not deleted...just moved.
Of course, someone who bookmarks their original thread may be shocked when it is no longer there, but still...it can be found...all they have to do is look for their most recent posts within their own profile(s).